Some SPOILERS here, just warning you.
Brolli was born at the same time as Goku and actually slept in a crib right next to his in a medical centre on planet Vegeta, only a little while before the planet was blown up. Because Brolli's birth powerlevel was so great, bigger than most average Saiya-Jins and over twice that of Vegeta's, King Vegeta order Brolli's death.
Both Brolli and his father were cast outside of the city like trash however when Frieza destroyed planet Vegeta, Brolli's strength created a Ki Shield which protected both him and his father.
After years and years Brolli became more and more unstable, forcing his father to employ a mind control device just to stop his mindless destruction. In the 1st Brolli Movie Brolli's father summons Vegeta to a planet he had taken control of, tricking him into thinking he was going to give him back the Saiya-Jin empire while in reality he only wanted to keep him there until a comet hit the planet, taking Vegeta and any chance of losing Brolli with it.
The problem was that Goku had severely affected Brolli as a child, his insecent crying drove Brolli out of his mind and just the sight of Goku caused Brolli to feel great rage. Somehow this rage allowed him to break free of his control and go after his revenge on Goku. That broke Brolli's father's plans, Brolli was unmasked and that led to the realisation of his father's plans.
And so ends the history of Brolli, I'll leave what happens at the end of the movie to your imagination since I don't want to spoil the whole thing.
