10 Villains Who Became X-Men
1. Magneto
Magneto needs little introduction - and not just because his name sums up his powers pretty well. He's tried to kill the X-men, tried to kidnap the X-men, ripped the Adamantium out of Wolverine and formed several iterations of the Brotherhood of Evil Mutants.
Following the events of House of M, Magneto's been pretty decent to the X-men. Given the largely reduced mutant population, he knows he must protect what little of his race that remains regardless of how many times he's fought them in past battles.
Although some X-men still don't fully trust him, Magneto has been an honourary member of the X-roster for quite a few years now. He currently leads X-men Blue, albeit from the shadows.
There was even a period where his powers were greatly reduced so he carried around a bag of metal nails and used them as crude bullets to help him hunt Nazis.
All together now: "Magneto was right."