What they're told.

Joffrey Baratheon was a cruel, sadistic, and entitled young man who was raised to believe that he was the rightful heir to the Iron Throne. He was spoiled and indulged by his mother, Cersei Lannister, and he enjoyed tormenting his younger siblings and subjects. When he ascended to the Iron Throne, he proved to be a cruel and incompetent ruler who executed Ned Stark and ordered the massacre of the Stark household. He was ultimately assassinated at his own wedding feast. Joffrey Baratheon was a complex and disturbing character. He was a product of his environment, but he was also responsible for his own actions. He was a reminder that even the most powerful people can be corrupted by power.