The X-Men have long had it harder than the other teams out there. They not only have to deal with normal superhero problems like supervillains, but they also have to deal with multiple groups trying to destroy their entire race, many of whom are funded by the most powerful corporate and governmental entities in the world. The fact that the X-Men have been able to hold onto their morals as a team for so long is quite impressive.
That said, some members have proven to be habitual line steppers. They've broken the team's rules more than others, sometimes for good reasons, but other times for the worst reasons imaginable.
10 Xorn Worked To Destroy The Team From Within
Xorn joined the X-Men during the New X-Men era of the team. His head became a star when his powers manifested, changing his life forever. He was found by the X-Men and freed from captivity, joining the team. At first, everything seemed great, but he started to suborn the team within, starting by not actually healing the Sentinel nanites in everyone.
Xorn turned the Special Class away from the rest of the faculty, convinced Esme Cuckoo to betray her teachers, and then pretended to be Magneto, attacking New York City. His final rule-breaking act was killing Jean Grey, but Wolverine made him pay for that with his life. He's since been resurrected on Krakoa.
9 Gambit Kept Secrets From The Team And Betrayed Them
Gambit came to the team with a lot of secrets, which in some ways was breaking the rules of the team. The X-Men are a family and secrets are frowned upon. His secret was a whopper, though. Gambit had helped the Marauders find the Morlocks tunnels, facilitating countless deaths. The reveal of this would cause him to leave the team for a time.
Later down the line, Gambit volunteered to become a Horseman of Apocalypse. Later still, he would work with Mister Sinister after returning to the team, cured of his Horseman powers. He proved to be untrustworthy several times, and it cost the team dearly.
8 Bishop Tried To Kill Hope Summers And Cable In Order To Stop His Future From Happening
Bishop came to the present from a blasted dystopian future and proved his worth with the team multiple times. However, cracks started to show in his facade. For example, Bishop ended up working with the pro-Registration forces during Civil War, breaking Cyclops's rule about working with either side. Later, he'd take a much more murderous stance against his teammates.
Unlike everyone else, the birth of Hope Summers wasn't a joyous event for Bishop. He believed that her birth was the catalyst for his terrible future, so he decided to kill her. This prompted Cable to take the baby through time, raising her as a soldier and keeping one step ahead of Bishop.
7 Cyclops's Leadership Of The Mutant Race Led Him To Break The Team's Rules
Cyclops is the pinnacle of the X-Men, but he hasn't always been perfect. This was made evident during his stewardship of the reduced mutant race after M-Day. Cyclops had to make a lot of tough decisions during this time, many of which went against the rules of the X-Men. He and Wolverine's creation of X-Force was a big one, as he sanctioned killing mutantkind's enemies, something he used to frown on.
Cyclops ordered Oya to kill the Hellfire Club's goons, something else that flew in the face of rules he once followed. He can be forgiven for his Phoenix Force-influenced extremes, but did work as a terrorist after that. Cyclops was willing to do whatever he could to protect his people, something that drove him places he never would've gone before.
6 Sabretooth Has Used The Team For His Own Ends Many Times
Sabretooth is a brutal murderer and has battled the X-Men many times over the years, although he has a specialization in tormenting Wolverine. One of the most paradoxical facets of the X-Men is that the team has always been willing to take in their foes as members with little proof they've actually reformed. Sabretooth has been a member of the team several times, and it never ended well.
Sabretooth is usually only doing it as a means to an end, ready to betray the team the first instance he can. He once even pretended to be a friendly lobotomized version of himself in order to escape captivity from the team, gravely injuring Psylocke. Sabretooth has no problem using the team whenever he wants to.
5 Emma Frost Never Met A Rule She Wouldn't Break
Emma Frost is a master manipulator, but she mostly gave that up when she joined the X-Men. Mostly. She egged Cyclops on to psychically cheat on Jean Grey with her, sowing division in the group. However, this would end with her and Cyclops getting together, something that was actually very good for him.
Emma worked with Norman Osborn and the Cabal, although she did eventually reveal that to Cyclops. She pretended Cyclops was alive after he died of M-Pox in order to implicate Black Bolt in his murder, then tricked the X-Men into attacking the Inhumans, even trying to use reprogrammed Sentinels against them before returning to the Hellfire Club. She's since come back to the X-Men.
4 Mystique Has Joined The Team Only To Betray Them
Mystique is among the deadliest mutants on the planet. She's been battling the X-Men for years, but she's also worked with them. For a while, she was secretly working for Xavier, but once that ended, she'd join the team as Foxx, all in a ploy to get Gambit to cheat on Rogue, so her foster daughter would leave him and the team.
Mystique later joined Rogue's rapid response team, but was secretly working with Mister Sinister the whole time, helping to destroy the X-Mansion. She also posed as Xavier and joined Norman Osborn's Dark X-Men. On Krakoa, Mystique broke the no precog rule, but she didn't technically know that was a rule.
3 Beast Endangered The Timestream And It Got Worse From There
Beast's intelligence has made him integral to the X-Men. However, as things for mutants got worse post-M-Day, he lost his moral compass. Beast endangered the entire timestream by bringing the original five X-Men to the present, but that was only the beginning. He then joined the Illuminati in Xavier's place and helped commit genocide on a massive scale during the Incursions.
Beast has since become the leader of the Krakoan X-Force, something that has seen him become even more amoral. He used telefloronic tech to destabilize an entire nation and covered up what he did. He's used his position in terrible ways, which is saying something for someone who leads a black ops organization.
2 Wolverine's Proclivity Towards Killing Had To Be Overlooked Many Times
Wolverine is the best there is at what he does. Oftentimes, that's killing people, which is a big no no, even on the X-Men. This mostly stopped Wolverine, but he was still known for killing on his adventures away from the X-Men, running through henchmen like there was no tomorrow. During the Utopia era, Wolverine and Cyclops started a new X-Force to secretly kill mutantkind's enemies.
Even after everyone learned the secret of the team, Wolverine still kept it up. Wolverine killed a lot of people over the years, and sometimes lied about it, so the amount of times he broke the team's rules is flabbergasting.
1 Professor X Made The Rules And Broke Them
Professor X is the first X-Man, having created the team. He made the rules of the group, which makes it all the more ironic that he would break so many of them. Xavier had no problems mind-controlling people, erasing memories, or lying to his students. He'd do anything to protect his people.
Xavier joined the Illuminati, kept the Danger Room enslaved after it became sentient, and covered up the deaths of an unknown X-Men team. Xavier was a big believer in "do as I say, not as I do," and his duplicitous nature would extend to Krakoa, as lying is basically what he does best.