Kinslaying is a great taboo in Westeros. When a member of family kills a relation he/she is dubbed a kinslayer. Any individual who slays a member of their own family is believed to be cursed forever in the sight of gods and men.  Many Lords and smallfolk believe this and thus those who slay their own blood are usually looked down upon or shunned. There is a saying... (old gods or new gods)
People accused of, or suspected of, or have committed kinslaying
- Stannis Baratheon – conceived a shadow assassin with Melisandre that went on to slay his brother King Renly Baratheon. His maester Cressen considers the thought of killing Renly
Fratricide . . . my lord, this is evil, unthinkable . .
- Tyrion Lannister: accused of the murder of his nephew, King Joffrey I, of which he's actually innocent. Also held responsible for the murder of his own father, Tywin Lannister, of which he is actually guilty.
- Gregor Clegane: is rumored to have murdered his own father and sister.
- Ramsay Bolton: his own father, Roose Bolton, suspects he poisoned his half-brother Domeric, Roose's trueborn son.
- Robb Stark: when preparing to execute Rickard Karstark, per Eddard Stark's custom that the man who passes the sentence should also swing the sword, Karstark calls Robb a kinslayer due to the (distant) blood ties between Starks and Karstarks.
- Theon Greyjoy: named a kinslayer by some for his supposed murder of Bran and Rickon Stark, as he was their father's ward, although they are not his blood kin.
Historical Kinslayers or suspected of Kinslaying
Brynden Rivers: Was called a Kinslayer when he killed his half-brother Daemon Blackfyre during the Battle of the Redgrass Field.
32pxMaelys Blackfyre: Was called a kinslayer because he supposedly consumed his own twin in the womb.
Maekar I Targaryen: Was called a kinslayer when he slew his own brother Baelor "Breakspear" Targaryen (although it was an accident).
32pxViserys II: Was suspected of poisoning his own nephew Baelor the Blessed. Though the evidence on this is dubious and it appears most likely that Viserys was innocent of this charge.
32pxKing Aegon II: For feeding his half sister Rhaenyra Targaryen to his dragon during the Dance of the Dragons.
Ser Erryk: For slaying his twin brother Ser Arryk during the Dance of the Dragons, both died by each others swords.
Ser Arryk: For slaying his twin brother Ser Erryk during the Dance of the Dragons, both died by each others swords.
32px Lord Stark (grandson of Brandon the Daughterless): Slew Bael the Bard, the King-Beyond-the-Wall, ignorant that he was his actual father.
- Customs for additional information on customs and traditions of Westeros.
The gods hate kinslayers, even when they kill unknowing. 
Kinslaying is dry work. It gives a man a thirst. 
The gods hate kinslayers. 
References and Notes
- A Clash of Kings, Chapter 44, Tyrion X.
- A Storm of Swords, Chapter 20, Catelyn III.
- A Feast for Crows, Chapter 18, The Iron Captain.
- A Feast for Crows, Chapter 29, The Reaver.
- A Clash of Kings, Prologue.
- A Storm of Swords, Chapter 70, Tyrion X.
- A Dance with Dragons, Chapter 27, Tyrion VII.
- A Dance with Dragons, Chapter 32, Reek III.
- A Dance with Dragons, Chapter 46, A Ghost in Winterfell.
- A Game of Thrones, Chapter 45, Eddard XII.
- A Clash of Kings, Chapter 51, Jon VI.
- A Clash of Kings, Chapter 51, Jon VI, p 545.
- A Dance with Dragons, Chapter 1, Tyrion I.
- A Dance with Dragons, Chapter 56, The Iron Suitor.