One thing that Professor X has always done with his X-Men team is to make it a place where even those who were former villains could find a home. This openness means there are often moments where villains have crossed the border between good and evil.
There is also the fact that many former X-Men members betrayed the team and went to the side of evil. When it comes to trust, the X-Men have significant issues because they bring in anyone and then have to hope the other shoe doesn't drop. Here is a look at 10 villains that everyone forgets were members of the X-Men.
Juggernaut had an immediate connection with the X-Men because he was the stepbrother of Charles Xavier. While Charles was kind and helpful, Cain Marko was cruel, spiteful, and bullied his stepbrother relentlessly. While Charles turned out to be a mutant, Juggernaut received his powers through a gem in the temple of Cyttorak.
With his new powers, Juggernaut became one of the X-Men's most dangerous villains. However, following the events of X-Men Forever and the battle with the sentient spaceship Prosh, Juggernaut became a hero and joined the X-Men. He eventually left for the Thunderbolts before going evil again.
When Mystique has worked with the X-Men, it has almost always been for nefarious personal reasons and she betrayed them many times. The first time she joined X-Men was when she was taken in by Wolverine after Freedom Force split up and she had a breakdown. She has even joined the X-Men in disguise, once again for selfish reasons.
The biggest problem with Mystique is that she kills. Her first time to join the X-Men ended when she tried to kill Graydon Creed. She also joined up later with Professor X as a double agent. This move eventually led to her joining the X-Men again, that time to remain close to her adopted daughter Rogue.
The reason Vulcan is a villain that everyone forgets was a member of the X-Men is because Professor X made us all forget. It was an intriguing move, as fans read in Giant-Sized X-Men #1 that an all-new X-Men team formed to save the original X-Men from a sentient island.
However, that was not the first rescue team Xavier put together. Everyone from the first team died except for Vulcan. Xavier then mindwiped anyone with knowledge and even mindwiped Vulcan's existence from his own family. That is a problem since he is Cyclops' brother, and that put a strain between Xavier and Scott when the truth came out.
Magneto has been a member of the X-Men almost as much as he has been their greatest enemy. Magneto was one of the first villains to show up in the world of the X-Men, leading the Brotherhood of Evil Mutants and quickly became their greatest villain.
However, when Professor X needed to leave Earth, he asked Magneto to take his place and teach his students the right way to use their powers. Amazingly, Magneto became a hero after this and even took on teaching duties of the New Mutants. His time would not last as he used his powers to exonerate himself in court for his past crimes. Since then, he has bounced back and forth more than almost anyone.
6 EMMA FROST
Emma Frost is the most recognizable name on this list as not only a member of the X-Men but also as one of their key leaders. Emma started as a member of the Hellfire Club as the White Queen before she opened her school, with rivals for the New Mutants in the Hellions.
When Emma Frost realized Charles Xaviers' methods of teaching were superior, they combined the two schools into one, and she became the Headmistress. Emma ended up in a relationship with Scott Summers and worked as a member of the X-Men. This partnership ended when she instigated war with The Inhumans and had to go on the run.
Xorn was an interesting character in the X-Men universe. His mutant power was possessing a small star in his brain, which he can manipulate the energy. However, it also burned away his face, and he had to wear a mask to protect himself. He ended up as a teacher for the school, and a good one too.
However, things went sideways when he started to teach Magneto's ideals to students instead of Xaviers. When he admitted to being Magneto in disguise, things went bad. The world believed Magneto died in Genosha, and after this, he led an attack on New York City.
Sabretooth was one of the X-Men's most powerful enemies, and a longtime Wolverine villain from the two mutant's childhoods. He worked with Magneto for most of his career as a villain, although he sided with Mystique when Magneto was on the other side of the playing field.
Sabretooth was forced to be a member of the X-Men when he helped them stop a race of superhumans known as The Children. Sadly, his hatred of Wolverine overpowered his sense of right and wrong.
3 LADY MASTERMIND
Lady Mastermind is Regan Wyngarde, the daughter of the original Mastermind. This makes it one of those cases where the X-Men are occasionally way too trusting. In this case, she joined the X-Men as an undercover sleeper agent for her father's old partner Sebastian Shaw.
Lady Mastermind finally struck and tried to kill Sage. The plan backfired, and when she recovered, she worked with Rogue's X-Men until revealing her long game. She betrayed them a second time, joining the Marauders before the Messiah Complex.
Much like Juggernaut, it was no surprise to see Legion as a member of the X-Men. However, just like his step-uncle, the son of Professor X could never be trusted. David Haller is a hero in his own mind's story, but his mind contains multiple personalities.
It wasn't just Shadow King controlling David either. Legion joined the X-Men at the decision of Cyclops, although Professor X was not sure it was a good idea. However, they needed him to beat the Nimrod Sentinels. Sadly, he could never control his personalities and never trusted the X-Men. He left the team for good when he decided he didn't like them training children as soldiers.
1 JAMIE MADROX
Jamie Madrox is a fascinating case. Thanks to the writing of Peter David, Madrox became one of the most interesting minor characters in the world of The X-Men. He initially wanted no part of joining the X-Men, before he finally agreed to join X-Factor.
Sadly, in the movie X-Men: The Last Stand, the film made some questionable decisions. The film turned characters like Psylocke and Jamie Madrox into villains. In the comics, he appears as a villain at times because he can't always control his duplicates, who are as likely to help a villain like Mr. Sinister.