Picollo is born as a result of Goku killing his father. Now his father was evil, but still Picollo wouldn't forgive Goku. He spent his life allone, and the only joy he had was his compitions with Goku. One day he hoped to destroy him for good.
At the beginin gof DBZ Picollo helps Goku to defeat Raddiz, swearing after he was done with Raddiz Goku was next. As luck would have it Picollo got them both in one blow, with a little asistance from a small boy with a tail. This boy of course was Goku's son Gohan. Picollo saw his amzing power and decide to train him. In the prosses, Picollo finaly made a friend and felt some thing besides hate.
When Vegia and Nappa arived the battle was very tough. Nappa fired a devistatin gblow in Gohan direction. But who would of thought that Picollo would have sacrifced himself to save Gohan. After this Picollo officaly beccame part of the Z team, and finnaly forgave Goku.