Kinslaying is the act of slaying a family member and a great taboo in Seven Kingdoms. Whoever commits it is dubbed a kinslayer. Any individual who slays a member of their own family is believed to be cursed forever in the sight of gods and men.
The taboo associated with kinslaying is strong enough to stay the hand of even the most ruthless men. Tywin Lannister would have liked nothing more than to kill his son Tyrion the day he was born, because his mother Joanna Lannister died giving birth to him and for the shame he brought on their family for being born a dwarf. However, Tywin felt that House Lannister was above the shame and curse of kinslaying, so he relented and let Tyrion live.
If a mother dies in childbirth, her offspring is not considered a kinslayer by the aforementioned faiths because there was no intent and it wasn't really their fault. The reactions of family members to the child can still vary, however, as sometimes they irrationally blame the child for the mother's death. Tywin always blamed Tyrion for "killing" his mother since she died giving birth to him, as did his sister Cersei Lannister. In contrast, his brother Jaime Lannister insisted that it wasn't his fault and always treated Tyrion with kindness and respect.
- King Maegor Targaryen committed two kinslayings:
- First, he killed his nephew Aegon.
- Later, at his command his other nephew Viserys was tortured to death.
- Prince Aemond Targaryen committed two kinslayings:
- Prince Daemon Targaryen committed two kinslayings:
- King Aegon II Targaryen committed two kinslayings:
- Arryk Cargyll and Erryk Cargyll, twin brothers who killed each other during The Dance of Dragons.
- Brynden Rivers, who killed his half-brother Daemon Blackfyre and two of his sons during the Battle of the Redgrass Field. Years later, he executed another of Daemon's sons. For this deed, he was stripped of his position as Hand of the King and sent to the Wall.
- Prince Maekar Targaryen, who accidentally dealt a mortal head injury to his brother, Prince Baelor Targaryen during the trial by seven of Ser Duncan the Tall.
- Orys I was killed by his unnamed brother.
- Robert Baratheon, who killed his distant maternal cousin Rhaegar Targaryen at the Battle of the Trident.
- Craster sacrificed his male offspring to the White Walkers.
- King Stannis Baratheon committed three kinslayings:
- Ser Jaime Lannister, who murders his distant cousin Alton Lannister in his attempt to escape captivity.
- King Robb Stark is accused of this by Lord Rickard Karstark, when the former is about to execute the latter for treason.
- Tyrion Lannister, who murders his father, Tywin Lannister. Tyrion later sarcastically quipped that because his mother Joanna died giving birth to him, from a technical standpoint he had "killed" both of his parents, making him the greatest Lannister-killer alive.
- Obara Sand, who murders her cousin, Trystane Martell, in a plot to take over Dorne.
- Ramsay Bolton, who murders his own father Lord Roose Bolton after the birth of his trueborn half-brother. Ramsay did this for fear that his father would disinherit him for his failures and name his newborn son by Walda as the new heir to House Bolton instead. He then proceeds to commit kinslaying a second time by turning his hunting dogs on his stepmother Walda and his own newborn half-brother, as Walda pleaded for mercy.
- Euron Greyjoy, who murders his brother, Balon Greyjoy, by throwing him off a rope bridge at Pyke in the middle of a storm.
- Cersei Lannister, who orchestrated the Destruction of the Great Sept of Baelor which killed her uncle Ser Kevan Lannister, her cousin Lancel, and, less egregiously, her daughter-in-law Margaery Tyrell.
- Sandor Clegane, who throws himself and his brother Ser Gregor Clegane off the Red Keep and into an inferno created by Drogon, during the Battle of King's Landing. However, as Gregor was technically a reanimated corpse, thus not being alive, this is debatable.
- Jon Snow, born Aegon Targaryen, who assassinates his aunt Daenerys Targaryen, to prevent more massacre after Daenerys laid waste to a surrendered King's Landing.
Alleged and suspected kinslayings
- Ser Gregor Clegane, suspected to have murdered his younger sister as well as his father.
- Ramsay Snow is suspected of murdering his trueborn half-brother Domeric Bolton, in order to further his position in House Bolton. It is also hinted that Ramsay had impregnated Violet, one of his bedwarmers, and had both her and their unborn child killed.
- Tyrion Lannister has been accused of poisoning his nephew King Joffrey Baratheon at his own wedding. The actual killer is Olenna Tyrell, with help from Petyr Baelish and Dontos Hollard, to spare her granddaughter Margaery Tyrell from enduring marriage to Joffrey.
- In 61 AC, King Jaehaerys I Targaryen and his former Hand Rogar Baratheon chased after Rogar's outlaw brother Borys. When the brothers confronted each other, Jaehaerys intervened, stating that he should fight the traitor, since he did not wish that his old friend would become a kinslayer; Borys scornfully responded "Rather name me a kingslayer than him a kinslayer!", charged at the king and was killed by him.
- Even though Rhaegar Targaryen had no love for killing according to his friend Ser Barristan Selmy, he might have been prepared to kill his distant cousin Robert Baratheon as a last resort to end Robert's Rebellion when the two of them faced each other in one-on-one combat at the Battle of the Trident, where Robert ended up killing him instead.
- Viserys Targaryen threatened to kill his sister and his unborn nephew Rhaego, in a futile attempt to force Drogo help him take back the Iron Throne, which instead led to his own death by Drogo.
- Randyll Tarly felt that his eldest son and heir Samwell was utterly unworthy to be his heir, so he forced Sam to join the Night's Watch and forswear all right to inheritance; he warned Sam that if he did not comply, he would take Sam out on a "hunting trip" and kill him in such a way that it would look like an accident. The terrified Sam subsequently joined the Watch. In view of Randyll's harsh personality and how much he despised Sam, it can be assumed it was not an idle threat, and he was ready to carry it out.
- During the Tourney of the Hand, Sandor Clegane and Gregor Clegane fight furiously over the latter's attempt to murder Loras Tyrell, and seem as if they will battle to the death due to their hatred for one another before Robert Baratheon orders them to stop. Several years later, during the Dragonpit Summit, Sandor confronts his brother and implies that he will eventually kill him.
- Stannis Baratheon was prepared to have Gendry, his bastard nephew by blood, burned alive as a blood sacrifice to the Lord of Light. He was thwarted by Davos Seaworth, who set Gendry free the night before.
- Tywin Lannister said that after Tyrion's mother died giving birth to him, he wanted to throw him into the ocean in his grief. He refrained from doing so because Tyrion is a Lannister.
- Cersei Lannister intended to kill her son Tommen out of mercy during the Battle of the Blackwater, in case Stannis conquered King's Landing, using the Essence of Nightshade she obtained from Pycelle. She nearly followed through with her plan, but her father saved the day when he entered the moment Cersei was about to administer the poison to Tommen, and declared that his army won the battle.
- Lysa Arryn attempted to kill her niece Sansa out of jealousy, after watching Littlefinger kiss her, by pushing her out the Moon Door; she was foiled by Littlefinger.
- Jaime Lannister vows to kill his brother Tyrion for murdering their father.
- Roose Bolton explains to his bastard son Ramsay that he fathered him by brutally raping a miller's wife. The miller had married without his consent as his lord, so Roose hanged the miller and raped the miller's wife under the same tree that the miller hung from. The next year, she showed up at the Dreadfort with the infant Ramsay, and Roose nearly had Ramsay thrown into the river. When Ramsay suggests that they attack the Night's Watch and kill Jon, an action that will unite the Northern Houses against the Boltons, Roose again threatens to have Ramsay killed if he refuses to behave.
- Nymeria Sand intends to kill her cousin Trystane, but Obara beats her to it.
- During the confrontation with Euron, Balon draws a knife to kill Euron, but gets killed.
- Smalljon Umber tells Ramsay that, had his own father not died on his own, he might have killed him himself.
- Realizing that her uncle Euron murdered her father Balon, Yara Greyjoy declared to all witnessed that once she won the Kingsmoot and the Salt Throne, her first order would be to execute Euron.
- Euron Greyjoy gives the order to have his niece and nephew, Yara and Theon Greyjoy, killed after he becomes the new King of the Iron Islands, but they flee the Iron Islands and take the Iron Fleet with them in anticipation of the danger.
- Rickard Karstark: "The blood of the First Men flows in my veins as much as yours, boy. I fought the Mad King for your father. I fought Joffrey for you. We are kin, Stark and Karstark."
- Robb Stark: "That didn't stop you from betraying me, and it won't save you now."
- Rickard Karstark: "I don't want it to save me. I want it to haunt you to the end of your days!"
- Robb Stark: "Kneel, my Lord. Rickard Karstark, Lord of Karhold, here in sight of Gods and men, I sentence you to die. Would you speak a final word?"
- Rickard Karstark: "Kill me and be cursed. You are no King of mine!"
- — Lord Rickard Karstark to King Robb Stark, his distant kin.[src]
- Davos Seaworth: "Forgive me, Your Grace, I'm not a learned man, but is there a difference between kill and sacrifice? The boy's your nephew."
- Stannis Baratheon: "What of it? We're at war. Why should I spare the son of some tavern slut Robert bedded one drunken night?"
- Davos Seaworth: "Because he has your blood in his veins."
- Stannis Baratheon: "So did Renly."
- Davos Seaworth: "Renly wronged you. Renly declared himself king when the throne belonged to you. He raised an army, stole your bannermen. This boy's done you no harm."
- — Davos Seaworth tries to dissuade Stannis from killing yet another member of his own family.[src]
- Tyrion Lannister: "What's the punishment for regicide? Drawing and quartering? Hanging? Breaking at the wheel?"
- Jaime Lannister: "Beheading."
- Tyrion Lannister: "Seems rather ordinary. And he was my nephew as well, so what's that? Fratricide is brothers, filicide is sons. Nepoticide! That's the one. Matricide, patricide, infanticide, suicide. There's no kind of killing that doesn't have its own word."
- Jaime Lannister: "Cousins."
- Tyrion Lannister: "Cousins? You're right. There is no word for cousin-killing! Well done."
- — Tyrion Lannister and Jaime Lannister discuss the various forms of familial killing.[src]
- "Tyrion may be a monster, but at least he killed our father on purpose. You killed him by mistake. With stupidity. You're a man of action, aren't you? When it occurs to you to do something, you do it. Never mind the consequences. Take a look. Look at the consequences. Here they are."
- ―Cersei Lannister to Jaime
- "He murdered my father. If I ever see him, I'll split him in two, and then I'll give him your regards."
- ―Jaime to Bronn
- "You want revenge against the Lannisters? I killed my mother, Joanna Lannister, on the day I was born. I killed my father, Tywin Lannister, with a bolt to the heart. I am the greatest Lannister killer of our time."
- ―Tyrion to Daenerys Targaryen
- Obara Sand: "We're not here to feed you, we're here to kill you."
- Nymeria Sand: "You want her to do it or me?"
- Trystane Martell: "We're family. I don't want to hurt you."
- — Obara and Nymeria Sand to Trystane Martell before they murder him.[src]
- Davos Seaworth: "I loved that girl like she was my own! She was good, she was kind and you killed her!"
- Melisandre: "So did her father. So did her mother."
- — Davos Seaworth and Melisandre about Shireen Baratheon's death.[src]
- Cersei Lannister: "I decline your proposal."
- Euron Greyjoy: "Why?"
- Cersei Lannister: "You're not trustworthy. You've broken promises to allies before and murdered them at the nearest opportunity. You murdered your own brother."
- Euron Greyjoy: "You should try it; it feels wonderful!"
- — Euron Greyjoy mocks Jaime Lannister whilst speaking with his sister Cersei.[src]
In the books
In the A Song of Ice and Fire novels, the taboo against kinslaying is often mentioned by various characters, among them Tyrion, Ygritte, Davos, Jaime, Rickard Karstark, Victarion Greyjoy, Roose Bolton, Jorah Mormont, Cregan Karstark, and Aeron Greyjoy.
It is pretty much the only reason that neither Tywin nor Tyrion ever try to kill Joffrey. Soon after Joffrey is crowned king it rapidly becomes apparent that he isn't simply vicious and cruel: he's a sadistic madman. Moreover, Joffrey is so utterly impulsive that he publicly, brazenly commits acts of petty cruelty that even the most ruthless kings of the past would have hesitated to commit, simply because it would be counter-productive. After Joffrey needlessly starts a city-wide riot, Bronn openly asks Tyrion why they don't simply assassinate Joffrey given that he causes far more problems than he solves, and moreover, actually has a pliable younger brother (Tommen) whom the Lannisters could easily use to replace him as their puppet king. Essentially the only reason Tyrion gives is that he won't kill his own nephew.
Jaime, hearing that Tyrion is charged with Joffrey's murder, muses that in the eyes of gods and men, kinslaying is considered to be even worse than kingslaying. He finds it difficult to believe his brother would commit such a crime. After learning the truth about Tysha, Tyrion is so angry that he tells Jaime (falsely) that he killed Joffrey, and shortly afterwards, he actually commits a kinslaying when he kills his father. Apparently, Tyrion is no longer bothered by the taboo because in the fifth novel, he remorselessly quips on several occasions that he killed his father and nephew, and even takes the "credit" for his mother's death; sometimes, Tyrion contemplates about killing his siblings too.
Characters in Westeros who have never met Daenerys, as they start to hear news of her string of conquests in Slaver's Bay, are sometimes appalled to hear that her husband Drogo killed her own brother Viserys as she watched. This is largely because they didn't know how much of a monster he was, or that he violated Dothraki law by drawing a sword in Vaes Dothrak and threatening Drogo's unborn son, and thus Drogo was entirely within his rights to execute him. Then again, in these discussions, other characters point out the well-known madness that runs in the Targaryen family, and accurately guess that Viserys might have been not much better than his crazed father King Aerys II Targaryen.
In the books, Stannis never admits that he ordered Melisandre to send the shadow demon against Renly, thus it is unclear if he is actually guilty of kinslaying. He was ready, though reluctantly, to have his bastard nephew Edric Storm sacrificed, but Davos prevented that kinslaying.
Theon Greyjoy is sometimes inaccurately accused of being a kinslayer for his alleged execution of Bran and Rickon Stark, though as a highborn hostage and ward of the Starks he was not actually their blood relative. He is referred to as Theon Turncloak.
Ramsay Bolton is suspected of poisoning his trueborn half-brother Domeric Bolton to take his place as Roose's heir, and Roose confides to Theon that he believes Ramsay will kill any trueborn child Roose has in the future, as the taboo of kinslaying means nothing to him. Though Domeric is never mentioned on the television show, he is mentioned by Ramsay in "Iron From Ice", the first episode of Game of Thrones: A Telltale Games Series.
"Alton Lannister" in the TV series is a renamed version of Cleos Frey from the books. Cleos's mother Genna is actually Tywin's sister, making Cleos a first cousin to Jaime. The TV series apparently renamed Cleos for fear that the audience would be confused why a man named "Frey" was fighting for the Lannisters in Season 2 (before the Freys turn on Robb Stark in Season 3), so they renamed him as a Lannister to make clear what side he is on. Jaime didn't kill Cleos Frey in the books, rather he accompanied Brienne of Tarth on the mission to return Jaime to King's Landing for a prisoner exchange, but he was killed by outlaw bandits along the way. The TV series later made clear in "A Man Without Honor", however, that "Alton Lannister" isn't exactly the same character as Cleos Frey, but is instead a far more distant cousin. Jaime has difficulty even remembering who Cleos's mother is, and Alton says she is Cynda Lannister, not Jaime's well-known aunt Genna. So on the one hand, Jaime was never a kinslayer in the books but the TV series has him kill Alton. On the other hand, the TV series changed the relationship of "Alton Lannister" so he actually isn't a very close relative of Jaime at all - it is debatable how much the shame of kinslaying would apply to a fifth or sixth cousin. Moreover, it's vaguely implied that "Alton" was willing to give his life in a heroic escape attempt if it would free the famous Jaime (though he didn't understand that when Jaime said Alton wouldn't survive the escape attempt, he meant that he would kill Alton to distract the guard).
Euron Greyjoy killed three of his brothers: Harlon, Robin and Balon. He is not brazen enough to admit that in public, only to Aeron whom he currently holds captive. Euron comments that Harlon and Robin were killed by his own hands, but Balon was not. Based on the prophecy of Balon's death, it is assumed Euron hired a Faceless Man for the assassination.
Victarion killed his pregnant wife (who was either raped or seduced by Euron), thus killing also his unborn nephew. He would have also killed Euron, but Balon forbade him due to the taboo of kinslaying.
Killing a relative by marriage, instead of a blood relative, is still considered kinslaying, because in a spiritual sense they have become part of the same family. It's possible that it isn't considered quite as bad but only by a matter of degree, i.e. killing your own brother or son is generally seen as somewhat worse than killing your own brother-in-law, though both are still reviled. Lord Tytos Blackwood's son was killed at the Red Wedding, and he notes that one of Lord Walder Frey's wives was herself a Blackwood, but apparently, the bonds of marriage mean nothing to Lord Walder. This wife was actually the mother of Lame Lothar Frey, meaning that Lame Lothar was responsible for the death of his kinsman Lucas Blackwood - given Lothar's club foot he might not have participated in the fighting directly, but he was one of the main masterminds who orchestrated the massacre and gave the orders. The exact relationship between Lame Lothar's Blackwood mother and the main Blackwood family hasn't been given, though the TV series might have condensed this. Just as kinslaying is considered a worse crime than regicide, violation of guest right is considered to be an even worse crime than kinslaying, so any kinslaying which occurred at the Red Wedding is overshadowed in the narrative by how outrageous and unthinkable it was that Lord Walder murdered guests under his own roof.
According to Ygritte, the gods hate kinslayers, even when they kill kin unknowingly. She told Jon a story about a wildling king named Bael the Bard, who kidnapped the daughter of one of the Stark lords, known as "Brandon the Daughterless". One day, the daughter returned with an infant, who eventually became the new Lord Stark. Thirty years later, the son fought against Bael (unaware of their kinship) and killed him. Bael recognized his son and allowed him to kill him, for he could not shed the blood of his kin. When Lord Stark returned from the battle and his mother saw Bael's head upon his spear, she threw herself from a tower in her grief. Her son did not long outlive her: one of his lords (presumably a Bolton) peeled the skin off him and wore it for a cloak.
Among the ironborn, it seems that the taboo is limited only to blood relatives, not relatives by marriage: Victarion Greyjoy wanted to kill Euron (for seducing/raping and impregnating Victarion's wife) but did not because Balon forbade him due to the taboo; no ironborn, however, stopped Victarion from killing his wife and unborn nephew, nor did anyone disapprove of that.
George R.R. Martin has said that there are considered to be different degrees of kinslaying, depending on the relationship and circumstances, though these may be unofficial. Degrees of kinship are taken into account: killing your sibling or parent are both reviled, and what is often thought of as "kinslaying", but killing a parent is considered the worse of the two. Killing an uncle or nephew is also viewed negatively (such as when Stannis pondered killing his bastard nephew Edric Storm, replaced with his other bastard nephew Gendry in the TV series). All of these variants are considered to be much worse than killing a distant cousin, though this is still frowned upon (Robert Baratheon killed Rhaegar Targaryen, even though they were second cousins, but Robert wasn't universally reviled for this). The aristocracy of the Seven Kingdoms is heavily intermarried and many succession wars centered around rival claims between different groups of distantly related relatives. Martin specifically explained that Rickard Karstark was "stretching" when he warned Robb Stark that if he executed him it would be considered kinslaying, given that the Karstarks branched off from the main Stark family around one thousand years ago.
On the fifth novel, Cregan Karstark tries to force Alys to marry him, but is imprisoned by Jon Snow. Similarly to Rickard, Cregan warns Jon that "Stark and Karstark are one blood", therefore killing him will make Jon a kinslayer. Cregan's claim is weaker than Rickard's, because Jon's name - as he points out - is Snow, not Stark. In that case, it does not really matter whether Cregan is correct, since Jon does not intend to execute him (not because of the kinslaying issue, but because the Watch does not take part in the quarrels of the realm).
It is unclear whether passively allowing a kin to be killed - rather than kill actively - can be considered as kinslaying. Jon, hearing that Axell Florent has done nothing to prevent his brother Alester's death, muses "If he is not a kinslayer, he is the next best thing... what sort of man can stand by idly and watch his own brother being burned alive?". If Jon is correct, Daenerys can be considered as kinslayer for doing nothing to prevent her brother's death; it seems unfair, though, that Viserys, who threatened to cut his pregnant sister open, should have been protected under the same taboo he was about to violate.
Martin also said that the circumstances of kinslaying are weighed by degrees (perhaps unofficially). Personally killing your own kin (with a sword, arrow, poison, etc.) is reviled, and hiring an assassin to specifically kill your kin for you is nearly as detested. If two brothers are leading rival armies against each other in war (such as Stannis and Renly), there are several variable outcomes. If Renly had announced to his army that he would reward the man who killed Stannis, or gave standing orders that Stannis was not to be taken alive, it would be viewed very negatively, though not quite as much as if Renly personally killed Stannis. If Renly publicly announced strict orders that he wanted Stannis captured alive, but Stannis accidentally ended up getting killed in the battle anyway, it would not be seen as nearly so negative. If Renly attacked Stannis's army but did not think to give specific orders about whether he wanted Stannis taken alive or dead, it would be seen as morally falling somewhere between the two, because he should have given more specific orders if he was concerned about avoiding kinslaying.
Therefore, personally murdering your own parent is considered to be the absolute worst kind of kinslaying - such as when Tyrion confronted his father Tywin with a crossbow and shot him dead. Robb Stark would, objectively, not be greatly blamed for executing Rickard Karstark, because he was a very distant cousin, and rather than "murder" Robb was within his rights to execute him for treason - though he chose to formally carry out the sentence with his own sword in hand. 
- "And Now His Watch Is Ended"
- "Robert's Rebellion (Histories & Lore)"
- "The Night Lands"
- "The Ghost of Harrenhal".
- "The Lion and the Rose".
- "The Dance of Dragons"
- "A Man Without Honor"
- "Kissed by Fire"
- "The Children"
- "The Red Woman"
- "The Iron Throne"
- "Unbowed, Unbent, Unbroken"
- "Kill the Boy"
- "Book of the Stranger"
- "The Door"
- So Spake Martin: Kinslaying in Westeros.