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.
Vegeta of the Dragon Ball Universe is probably the most famous example of a villain joining the good guys. When Vegeta first appears, he's a vicious murderer who wants nothing more than to defeat Goku, known to Vegeta by his Saiyan name, Kakarot. After being forced to work with Goku and his friends toward their common goal of defeating Frieza, Vegeta finds himself protecting his former enemies, and being protected in return. When he gets hurt and is forced to stay on Earth longer than he meant to, he starts getting used to the people there, and even growing attached to them.
It takes several relapses into his old ways to fully change, but eventually he does become a valued and trusted ally to Goku and his friends.
In Naruto, most villains eventually realize the error of their ways. However, most of them die before they get a chance to join the good guys. Gaara is one of the exceptions.
When Gaara first appears, he's hellbent on killing as many people as he possibly can, referring to them as prey. This is a trauma reaction - like Naruto, he was vilified by his people for having a monster sealed inside of his body, but unlike Naruto, he was physically attacked by his own family. His father and uncle literally tried to murder him repeatedly. After a protracted fight with Team 7, Naruto and Gaara have a heart to heart talk about their mutual feelings of isolation and pain, and this convinces Gaara to stop killing people and dedicate himself to protecting them, instead.
Yu Yu Hakusho features the murderous Hiei, who dedicated his childhood to various killing sprees. He had to join the Spirit Detectives as a condition of his parole, and he was pretty surly about it until they took on a mission to rescue his twin sister Yukina, whose relation to him had been kept a secret. Despite not being being forced to accompany them this time, Hiei still tags along, making up excuses for why he's interested the whole time.
Scar is probably one of the most well-developed characters in Fullmetal Alchemist: Brotherhood. He starts off as a serial killer who targets State Alchemists. Without context, this makes him seem like a monster - with context, it gets more complicated. State Alchemists nearly wiped out his people, the Ishvalans, and destroyed their homeland. Despite belonging to a religion that preaches peace, Scar considers genocide to be a worthy reason for revenge.
His rage leads him to kill people who were trying to help him, including the Rockbells, who were Amestrian doctors attempting to treat his wounds. When he's confronted with the Rockbell's daughter Winry, who hates him for murdering her parents, but is still willing to help him because that's what her parents would have wanted, he must take stock of the impact of his revenge. As the series progresses, he puts his justified feelings aside to ally with Al, Ed, and others who are working toward the same thing he is - peace.