These antagonists are rarely the Big Bad. Main villains who "turn over a new leaf" and join the hero at the end usually only happens in media for very young audiences (while hitting us over the head with a ~moral~). No, these guys are usually either the villain sidekicks or the red herring villains.
What reasons might a villain switch sides?
• an acceptance of moral responsibility
• to spite the main antagonist
• for love (family or romantic)
The only character who comes to mind immediately is Zuko from Avatar: The Last Airbender. He was the main antagonist in season one, although, even then, he displayed some moral integrity, choosing to save his Uncle instead of pursuing the Avatar and there was a lot of focus on his complicated past. But for season two and half of season three, the main antagonist switches to his sister Azula (and father the Fire Lord) while Zuko struggles between the destiny he thinks he should have and the destiny that's truly awaiting him. He's my favorite character in Avatar :D
I think the Malfoys might also fall into this category. They oppose Harry the entire series until the end when Narcissa lies to Voldemort in order to get back to her son. Harry even saves Draco at one point, and I'm pretty sure that was a symbolic olive branch.
What do you guys think about these antagonists? Love them? Hate them?
Have a great Monday! ♥