Oathbreaking

"So many vows. They make you swear and swear... Defend the King, obey the King, obey your father, protect the innocent, defend the weak. But what if your father despises the King? What if the King massacres the innocent? It's too much. No matter what you do you're forsaking one vow or another."

- Ser Jaime Lannister

Oathbreaking, the direct violation of an oath before the gods, is a crime in Westeros, just as serious as as regicide - which can be considered oathbreaking - or kinslaying. Oathbreaking is punishable by death.

Cases of Oathbreaking

 * Ser Jaime Lannister, who betrayed his oaths as a Kingsguard and slew King Aerys II Targaryen during the Sack of King's Landing to prevent him from carrying out his Wildfire plot, earning the nickname of "Kingslayer". Pardoned by King Robert Baratheon.
 * Mance Rayder, who deserted his post as a Ranger of the Night's Watch to live among the Free Folk and install himself as King-beyond-the-Wall
 * {Will}, a Ranger of the Night's Watch, deserted his post after witnessing an attack by the White Walkers. Beheaded by Ned Stark.
 * The Betrayers, led by Rast and Karl, who mutinied against Lord Commander Jeor Mormont