From A Wiki of Ice and Fire - A Song of Ice and Fire & Game of Thrones
Kingslayer is another name for Ser Jaime Lannister. He earned the name when, as a member of the Kingsguard, he slew the king he was supposed to be guarding, Aerys II Targaryen, on the steps before the Iron Throne.
|“||The Kingslayer. The false knight who profaned his blade with the blood of the king he had sworn to defend.||”|
|“||Kingslayer. And such a king he was! To Aerys Targaryen, the Second of His Name, Lord of the Seven Kingdoms and Protector of the Realm. And to the sword that opened his throat. A golden sword, don't you know. Until his blood ran red down the blade. Those are the Lannister colors, red and gold.||”|