Kingslaying is one of the great taboo in Westeros. When a sworn lord, lady or knight broke the vow and killed the king, crime to be processed. It is not related to killing a king who didn't accept his kingship and didn't make oath to him.
People accused of, or suspected of, having committed Kingslaying
- Walder Frey: Killed his king Robb Stark at the Red Wedding, which he committed guest right violation too. 
- Petyr Baelish: Killed his king Joffrey with Olenna at Purple Wedding. Which Tyrion and Sansa accused of. 
Known or suspected victims of kingslaying
- Ser Oswell Whent
The Kingslayer...The false knight who profaned his blade with the blood of the king he had sworn to defend.
- Roose Bolton when killing his king.
References and Notes
- A Storm of Swords, Chapter 11, Jaime II.
- A Storm of Swords, Chapter 51, Catelyn VII.
- A Storm of Swords, Chapter 68, Sansa VI.
- A Feast for Crows, Chapter 18, The Iron Captain.
- A Game of Thrones, Chapter 39, Eddard X, p 425.
- A Game of Thrones, Chapter 57, Sansa V.
- A Game of Thrones, Chapter 51, Sansa IV.