King Broly was short-tempered and violent, but that was very accepted in Saiyan society.
Sometimes, Broly turned Super Saiyan, but he was never really challenged. So the Legendary form stayed dormant, and he remained (quite) sane.
The young genius Vegeta studied the King with extreme jealousy.
At first, Vegeta was in fear, then in anger, then in hate.
...then, in suffering.
Aaaaaahhh!!