He is one of the characters who was originally "evil", but through series of unexpected events, the "evil" in him was slowly washed away as Piccolo took different forms.
Piccolo is the offspring of Daimon, who was the pure "evil" which separated from Kami-sama as another body. Piccolo and Kami-sama is one person in two bodies. If one of them die, both will die.
When Daimon was killed by Son Goku, he vowed that his offspring Piccolo will avenge him. Thus, Piccolo's character is not just pure "evil" the way Daimon is. He is simply focused on destroying Son Goku. The two of them first met in the Budokai.
Because of the arrival of Raditz, Piccolo found himself an unwilling partner of Son Goku and together defeated Raditz. When he learned that two more poweful Saiyajin will avenge Raditz, Piccolo only found himself even more unwillingly on Goku's side by volunteering to train his Son, Gohan to become a warrior to fight against the Saiyajin.
Piccolo's militant training style made Gohan a stronger person, gradually unleashing Gohan's tremendous talent. During the time he spent with Gohan, Piccolo began to develop love and care for Gohan, which was fully realized when he sacrificed his life for Gohan in the battle against Nappa. His love and care for Gohan is readily apparent throughout the Freeza and Cell series.
