Regicide

Jaime Lannister murdered Aerys II Targaryen during the Sack of King's Landing, earning him the derogatory nickname of "Kingslayer".

"Is that what you tell yourself at night? That you're a servant of justice? That you were avenging my father when you shoved your sword in Aerys Targaryen's back?"

―Eddard Stark to Jaime Lannister

Regicide, also known as kingslaying, (feminine equivalent presumably being queenslaying) is the deliberate act of murdering a monarch and is considered one of the most serious and stigmatized crimes in the Seven Kingdoms. Whoever commits it is dubbed a kingslayer. It usually refers to the act of killing one's own king, making the act a form of oathbreaking. Killing a king in battle is generally not considered kingslaying, particularly if the dead king is a rival to one's own or an otherwise declared enemy.

Somewhat like kinslaying, incest, or violating guest right, anyone who kills a king is believed to be cursed. Because the King of the Andals and the First Men is blessed by the High Septon of the Faith of the Seven at his coronation, it is considered a heinous crime in that religion to kill the king. This applies even when the king is a reviled tyrant.

Known kingslayers

"Perhaps you should speak to me more softly then. Monsters are dangerous - and just now, kings are dying like flies."

―Tyrion Lannister to King Joffrey Baratheon after the Red Wedding.

Roose Bolton murders Robb Stark during the Red Wedding.

Renly Baratheon, self-proclaimed king, is killed by a shadow monster created by Stannis Baratheon and Melisandre.

Brienne of Tarth pulls out her sword Oathkeeper, minutes before killing Stannis.

• King Orys I was killed by his brother.
• King Aegon I Targaryen, who attacked Harrenhal with dragonfire during the Burning of Harrenhal, thereby roasting King Harren Hoare alive.
• Lord Orys Baratheon, who slew King Argilac Durrandon during the Last Storm.
• King Aegon II Targaryen, who ordered his dragon, Sunfyre, to burn his half-sister Queen Rhaenyra Targaryen to death.
  • Aegon himself was later poisoned by his own men, making them kingslayers. 
• Ser Jaime Lannister, perhaps the most notable example of a kingslayer, who killed Aerys II Targaryen during the Sack of King's Landing. Jaime's actions were considered especially heinous, as he was, in fact, a member of Aerys's own Kingsguard, and took a holy vow to lay down his life in defense of his king. Jaime actually killed Aerys in order to foil his scheme to destroy King's Landing, but since he never told anyone the truth (with the recent exception of Brienne) - everyone assumed he killed Aerys so the Lannisters would seize the throne. It is unknown whether Jaime would still bear the stigma of kingslayer if he revealed his real motive, particularly since there are only a few who would even believe him, and others wouldn't care what was the reason. He committed a second kingslaying by killing King Euron Greyjoy during the Battle of King's Landing.
• Queen Cersei Lannister, along with her cousin Lancel, conspired to kill King Robert Baratheon. At Cersei's orders, Lancel gave Robert a strong wine, thus dulling his reflexes, and ultimately leading him to suffer a fatal wound upon a boar's tusk.
• King Stannis Baratheon, who conceives a shadow assassin with Melisandre to kill King Renly. This may not be considered a true example of kingslaying, as Renly himself was a usurper, and was in open rebellion against his elder brother, the rightful king, who in turn was in rebellion against the nominal King on the Iron Throne. Renly never sat upon the Iron Throne, not even coming particularly close to doing so.
• Lord Roose Bolton betrayed and personally murdered his own King, Robb Stark, during the massacre known as the "Red Wedding". The massacre occurred at the Twins under the roof of Lord Walder Frey, who joined Roose in organizing the betrayal, and whose crossbowmen wounded Robb at the beginning of the attack. Lord Tywin Lannister also helped plan the betrayal from a distance, as he guaranteed the Freys and Boltons that they would not be punished for breaking guest right. Still, while others such as Walder or Tywin share in the guilt, it was Roose himself who struck the deathblow. For his part in Robb's death, Walder Frey considers himself a kingslayer, which in his twisted psyche is a source of pride rather than shame.
• Lord Petyr Baelish and Lady Olenna Tyrell, who assassinated King Joffrey Baratheon, with poisoned wine at his wedding. 
• Jon Snow shot Mance Rayder, the King-Beyond-the-Wall, to spare him from being burned alive. He committed a second regicide by killing Queen Daenerys Targaryen to save Westeros after she massacres the population of King's Landing.   
• Ser Brienne of Tarth became a kingslayer when she assassinated an unarmed and wounded King Stannis Baratheon after his defeat at the Battle of Winterfell in order to avenge Renly's death.
• King Euron Greyjoy murdered his elder brother, King Balon Greyjoy, by throwing him from a bridge.
• Queen Daenerys Targaryen, who attacked the Red Keep with dragonfire during the Battle of King's Landing, thereby causing debris to crush Queen Cersei Lannister to death.

Alleged and suspected kingslayers

• Brienne of Tarth has been wrongly held suspect for Renly's murder.
• Tyrion Lannister and Sansa Stark were falsely accused of assassinating King Joffrey.

Contemplated and would-be kingslayers

• It is implied that Varys, using one of his little birds, attempted to poison Queen Daenerys Targaryen prior to the attack of King's Landing, to prevent the slaughter of innocent lives. His plot was foiled because Daenerys wouldn't eat anything, consumed by grief over the deaths of Rhaegal and Missandei.
• Arya Stark had plans to kill Queen Cersei Lannister, but was eventually dissuaded by the Hound.
• Sansa Stark was about to push King Joffrey off the ramparts of Traitor's Walk in the Red Keep, as a payback for her father's death. She was stopped by the Hound before she could do this.
• King Stannis Baratheon and Melisandre sought to kill the kings Robb, Balon and Joffrey by performing a ritual of blood magic. It is doubtful the ritual had anything to do with the deaths of the intended victims.

Quotes

"A man who profaned his blade with the blood of the King he had sworn to defend!"

―Barristan Selmy on Jaime Lannister the Kingslayer.

"Kingslayer! And what a king he was! Here's to Aerys Targaryen, the second of his name, Lord of the Seven Kingdoms, Protector of the Realm, and to the sword I shoved in his back!"

―Jaime Lannister on his killing of Aerys II Targaryen.

"The Kingslayer Brothers. You like it?"

―Tyrion Lannister to his brother Jaime.

Cersei Lannister: "Roose Bolton is Warden of the North by the grace of my father."

Petyr Baelish: "Indeed. His reward for stabbing his own king in the heart."

— Cersei Lannister and Petyr Baelish

Petyr Baelish: "I promise you..."

Olenna Tyrell: "I promise you, Lord Baelish, that our fates are joined. Together, we murdered a king."

— Olenna Tyrell reminds Petyr Baelish that they are kingslayers.

Jaime Lannister: "I'm sorry if this conflict has inconvenienced you, but rebelling against the Crown has consequences."

Edmure Tully: "Says the man who shoved his sword through the king's back!"

— Jaime spars with Edmure Tully.

"You talk about war as if you're an expert, but the one battle I remember you fighting, you were captured by Robb Stark, the Young Wolf. But it doesn't matter. Here we are now, two kingslayers."

―Walder Frey to Jaime

In the books

In the A Song of Ice and Fire novels, the taboo against kingslaying is much the same.

Jaime Lannister directly states that in the eyes of gods and men, kinslaying is considered to be even worse than regicide - thus while men often think of him as honorless and call him "the Kingslayer", Jaime still has major personal reservations against kinslaying, which would make him sink even lower on the moral scale. Unlike Cersei, Jaime actually never liked Joffrey and recognized him for the sociopathic monster that he is. Having already killed one king, and been made a social pariah for it, Jaime no longer had any personal restrictions on simply killing another king. Even so, Jaime never considered killing Joffrey to remove him from the line of succession, not because he was a king, but because if nothing else, Jaime would not kill his own son.

See also

• Kingslaying on A Wiki of Ice and Fire (MAJOR spoilers from the books)
• Regicide on Wikipedia

References

Template:Crime and Punishment