The X-Men have faced off against some of the most traitorous villains in Marvel Comics history, including Apocalypse and Mr. Sinister. But often, thanks to the complex landscape of mutant-human relations and differing views on the best way to achieve peaceful results, the X-Men have turned on each other.
Some of the most devastating betrayals the X-Men have suffered in Marvel Comics have been on of their own rank and file. Sometimes their reasoning was justified, and sometimes it wasn't. Many times it led to catastrophic consequences for the entire team and sometimes even the entire world.
Colossus
Colossus is generally one of the most steadfast of the X-Men, so it was a huge shock when he turned his back on them. Colossus betrayed the X-Men and ed Magneto's Acolytes in the 90s, an act largely motivated by his disillusionment after the death of his sister Ilyana to the Legacy Virus.
Colossus fought against his former teammates on a few occasions, proving how fortunate it was for them to have him on his side in the first place. Colossus eventually soured on Magneto's increasingly brazen actions and came back to the fold.
Wolverine
Wolverine has betrayed the X-Men on a number of occasions, for different reasons. One of the worst examples comes from the Old Man Logan universe, one of Marvel Comics' darkest future timelines.
In this alternate reality, Mysterio tricks Wolverine into killing all of the X-Men. Logan has betrayed the X-Men in other realities as well, including in an issue of the What If..? comic book. In that story, he's turned into a vampire and then proceeds to feed on his teammates, including the woman he loves, Jean Grey.
Mystique
Mystique has traditionally been a villain of the X-Men, so her betrayals aren't entirely unexpected. But in the Krakoan era, she is a member of the White Council and on the same side as all mutants. Her recent actions in the Inferno mini-series make it clear she is betraying the vision of Krakoa and the will of Moira MacTaggert, even if Mystique doesn't know all the details.
She is also one of the biggest suspects in the murder of the Scarlet Witch, which could potentially be an even greater betrayal of mutant edicts depending on if Wanda is retconned to be a mutant again.
Gambit
Gambit has always been a slippery character from his introduction in the 80s, which made sense as he was a product of the thieve's guild. His minor transgressions weren't entirely unexpected, but his betrayal of the X-Men to become one of the Horsemen of Apocalypse was unforgivable.
To his credit, Gambit wasn't entirely in control when Apocalypse transformed him into Death, but he didn't exactly resist the opportunity for greater power either. It was the worst perhaps in a long line of acts that make Gambit hard to trust.
Jean Grey
Jean Grey wasn't really in control of her own actions during The Dark Phoenix Saga, which provided some of the best X-Men comic book issues of the 70s. Readers didn't know that at the time, so it was shocking to see Jean Grey indulge her lust for power and control to such cosmic lengths.
She fought her team and the entire Shi'ar Empire to feed her stellar hunger, resulting in the deaths of billions of innocent people. Jean Grey ultimately sacrificed her life rather than lose control completely, ending one of the best X-Men stories ever.
Angel
Gambit isn't the only X-Man to turn to Apocalypse. Angel also became one of the Horsemen of Apocalypse, transforming into the dark vision of Archangel, in some of the best X-Factor issues ever.
Archangel also had limited control in what happened to him, but Warren Worthington nevertheless played some part in turning on his teammates and the world. His guilt and despair would motivate him in later years once he returned to his heroic form, but the shadow of Archangel still hangs over Warren and his teammates to this day.
Cyclops
One of the greatest betrayals in Marvel history occurred when Cyclops murdered Professor X in the final moments of Avengers Vs. X-Men. The shocking twist of the perfect student killing his beloved mentor was a bitter rebuke of everything the two characters ever stood for.
Cyclops was possessed by a fragment of the Phoenix Force at the time, but his actions were his own. He would go on to become a much more extremist figure, in the mold of Magneto, though his most recent iteration has largely restored him to the good, loyal soldier he has been in the past.
Professor X
Professor X himself has betrayed the X-Men and his own vision. In the 90s, Professor X's mind entangled with the consciousness of Magneto to become Onslaught. Onslaught betrayed the X-Men and tried to destroy them, and almost succeeded in destroying the Avengers and Fantastic Four.
Onslaught remains a threat on Krakoa today, a spectral reminder that Charles Xavier's actions in the current comics are arguably not in keeping with his original vision of humans and mutants living together in peace.
Magneto
Magneto is one of the X-Men's greatest villains, but he's also been one of their closest allies. For a long time in the 80s, Magneto was actually the head of Xavier's School For Gifted Youth in Professor X's absence. He fought alongside the team in major battles but betrayed them when he went back to his villainous ways in the 90s.
During this period, he ripped all the adamantium out of Wolverine's skeleton. He may also be working against them on Krakoa, though given what's been revealed, it's unlikely he is the person who murdered the Scarlet Witch at the Hellfire Gala.
Scarlet Witch
One of the motivating factors in Wanda's murder may have been her actions in House Of M. Though she has since been retconned to not be a mutant, at the time, her erasure of the vast majority of mutants from existence was a betrayal of incalculable magnitude.
The Scarlet Witch nearly drove her people to extinction and left them vulnerable to other threats. Mutants are just now recovering on Krakoa, but their hate of Wanda has never abated, reflected in the callous way they still refer to her as 'The Great Pretender,' even in death.
