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human
[Dragonball] How does Picollo become a good guy?

In the original Dragonball, Picollo was literally Kame's evil side; however, after training Gohan to prepare him for the Sayans, he became good. How exactly is that possible?
gpt
A few reasons:

1: The Piccolo that turned good was the evil Piccolo reborn.  As we saw with Buu (and as those educated in the various religions that involve reincarnation would know), while you carry the burden of a previous life's sins, you aren't doomed to follow the same path.

2: As Kami stated, Piccolo was aware of his impending death, and brushes with death have been known to change people.

3: Piccolo actually achieved what he set out to do- kill Goku.  After that, he realized that he felt empty inside rather than elated.  Granted, he had wanted to win in 1v1 combat but he isn't a proud-warrior-race guy, he didn't mind being sneaky or underhanded in order to win, and even flat out stated he'd turn on Goku after they defeated his brother.

4: He was also impressed by Goku's sacrifice

5: He also knew the Saiyans were coming, and were even more powerful than Radditz.

6: In addition to Gohan being pure of heart, it's been scientifically proven that being stuck with someone in a traumatic situation over time can cause bonding.

7: Once he sacrificed himself for Gohan, he was pretty much set on the path to goodness.  Re-discovering his roots on Namek and fighting for the cause of his people was just the icing on the cake.