Anime villains don't always stay villainous. Actually, there are plenty of anime villains who turned good. In series like Naruto and Yu-Gi-Oh! this happens to nearly every villain, while other anime use the trope more sparingly. Sometimes it's heartfelt, and sometimes it's kind of hamfisted, and it happens for a lot of different reasons.
In some cases, a change of heart occurs when an intelligent villain realizes that their former behavior is no longer in their best interest. In other cases, a villain with good intentions finally gets the opportunity to be the good guy they've always been on the inside. Maybe a villain who was really into the whole evil thing realizes the error of their ways after a showdown with the protagonist. Maybe they begrudgingly join the good guys, but their old personality keeps creeping in. No matter the reason, characters who make the switch from villain to good guy can be fascinating.
Which switchover did you think was the most plausible and interesting? Vote up the anime villain who joined the good guys whose story most resonated with you, and vote down the characters who would have been better off staying villains.