Think of it this way Sean, Son Goku is a good person, but in order to keep the world safe he has to turn into a Super Saiyan and kick ass. If he didn't all the villains that could destroy the Earth or the entire Universe for that matter (Freeza, Cell, Majin Buu) would have won and caused destruction everywhere if Goku just sat around eating and not fighting.

Genki Dama