...piccolo was evil until the end of the first episode/movie of dbZ. When the first saiyan came to earth Goku and Piccolo teamed up to kill him, but weren't powerful enough to stop him. So Goku jumped on Raditz' (the saiyan) back and held him down long enough for Piccolo to charge up his new attack, the Special Beam Cannon. After 5 minutes of charging, Piccolo fired his Special Beam Cannon at Raditz but since Goku was on his back, it killed Goku along with Raditz. Piccolo thought "Wow, here's a man who is prepared to die for the life of his enemies. I need to be nice now" or something along those lines, then Piccolo took Goku's first son, Gohan (a young boy at the time) to the wilderness for intense training.
Dragon Ball Z is the shit.