With Piccolo, Tien, Vegeta, and even Buu, Akira Toriyama has a way of giving his greatest villains a change-of-heart and turning them into heroes. This is a reoccurring theme that occurs all throughout the Dragon Ball franchise, from the original Dragon Ball series up until Super's latest arcs in the manga, and it shows no signs of stopping any time soon. Toriyama is seemingly obsessed with turning terrible villains into virtuous heroes.
Source：https://www.cbr.com/db-villains-to-heroes/
