Regicide

"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, or 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 and alleged 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.


 * 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. He became a kingslayer for the second time when he killed King Euron Greyjoy during the Battle of King's Landing.
 * Queen Cersei Lannister, along with her cousin Lancel, conspired to bring about the death of her husband King Robert Baratheon. On Cersei's instruction, Lancel replaced the wine in the king's flask with a stronger, fortified vintage, causing Robert to quickly descend into an intoxicated state, 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 his brother 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.
 * Ser Brienne of Tarth, who had sworn herself to Renly as one of his Kingsguard, is also held suspect for the murder, and is thus accused of being a kingslayer.
 * 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 Tyrion Lannister stood accused of assassinating his own nephew, King Joffrey Baratheon, with poisoned wine at his own wedding, although the charges were utterly groundless and leveled against him solely by Cersei.
 * The actual kingslayer is Lady Olenna Tyrell who carried out the actual poisoning, and plotted it with Lord Petyr Baelish.
 * Ser Brienne of Tarth became a true kingslayer when she assassinated an unarmed and wounded King Stannis Baratheon after his defeat at the Battle of Winterfell.
 * King Euron Greyjoy, who murders his elder brother, King Balon Greyjoy, 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.
 * Lord Orys Baratheon, who slew King Argilac Durrandon during the War of Conquest.

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.

"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.