From A Wiki of Ice and Fire
Kingslayer is another name for Ser Jaime Lannister. He earned the name when, as a member of the Kingsguard, he slew his king, Aerys II on the steps before the Iron Throne, the king he was supposed to be guarding.
See also ; Kingslaying
|“||"Kingslayer," he pronounced carefully. "And such a king he was! To Aerys Targaryen... and to the sword that opened his throat. A golden sword, don't you know. Those are the Lannister colors, red and gold."||”|