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

Kingslaying, also known as regicide, is the deliberate act of murdering a monarch and is considered the most severe crime in the Seven Kingdoms. Whoever commits it is dubbed a kingslayer.

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 who sits the Iron Throne.

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' own Kingsguard, and took a holy vow to lay down his life in defence of his king Jaime killed Aerys in order to foil his scheme to destroy King's Landing, but since he never revealed it to anyone (except recently to 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.
 * Khal Drogo murdered Viserys Targaryen by pouring molten gold onto his head. Despite technically being the rightful king of Westeros, Viserys himself was an exile at the time, and never actually sat upon the Iron Throne.
 * 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. Moreover, Renly was never officially sat upon the Iron Throne.
 * Lord Roose Bolton betrayed and murdered his own King, Robb Stark, during the massacre known as the "Red Wedding".

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