Kingslaying is the act of killing one's own king, breaking their vows of allegiance to him. In Westeros, kingslaying is regarded as a exxtremely vile act, especially when commited by someone sworn to protect them, as is the case with the Kingsguard. It should not be confused with killing a king to which one has no affiliation.
People accused of, or suspected of, having committed kingslaying
- Euron Greyjoy is suspected of being involved in the death of his brother and king Balon Greyjoy, which would also qualify as kinslaying. 
- Jaime Lannister killed his king Aerys II, despite being a sworn brother of the Kingsguard at the time. 
- Olenna Redwyne killed her king Joffrey with the help of Petyr Baelish and managed not to be part of the suspects, while the blame fell to Tyrion and Sansa. 
- Roose Bolton stabbed his king through the heart at the Red Wedding, which he planned alongisde Walder Frey, who violated guest right with the same act. 
Known or suspected victims of kingslaying
Quotes about kingslaying
“The Kingslayer,” Ser Barristan said, his voice hard with contempt. “The false knight who profaned his blade with the blood of the king he had sworn to defend.”
“Jaime Lannister sends his regards.”
- Roose Bolton when killing his king.