The X-Men -- a team of mutant heroes formed by Professor Charles Xavier -- are one of the world's most popular superhero teams. Pledged to protect a world that both hates and fears them, the X-Men are traditionally a force for good. Even so, out of their five original members, four have at some time or another acted in ways that were not heroic -- and the same can be said of their mentor Professor X.
With the sole exception of Iceman, the original X-Men have either resorted to villainous acts or at the least been the reflection of villainy. Following is a list of the not so heroic acts that have been committed by the inaugural team of X-Men.
Jean Grey / Dark Phoenix
Perhaps the most well-known occurrence of an X-Men breaking bad happened in Chris Claremont and John Byrne's Dark Phoenix Saga, presented in 1980's Uncanny X-Men 129-138.
Largely revered as a classic and recognized as one of the best comic book sagas of all time, the Dark Phoenix Saga tells the story of Jean Grey's corruption by the cosmic entity known as the Phoenix Force, her catastrophic destruction of a star system that killed billions and her subsequent death as retribution for her acts. The story has been depicted on X-Men: The Animated Series and on the big screen in X-Men: Dark Phoenix.
The story was retconned years later, with the explanation that the evil force was in actuality the Phoenix Force itself masquerading as Jean, with the young mutant placed safely in catatonic rest in a cocoon hidden at the bottom of the ocean. Unfortunately, Jean was once again the face of evil when Mister Sinister created a clone of her, Madelyne Pryor, who not only was able to temporarily harness the Phoenix Force but also became the evil Goblin Queen.
Angel / Archangel
Nearly as popular as Jean Grey's metamorphosis into Phoenix is the transformation of Warren Worthington III, aka Angel, into Death -- the blue-skinned, razor-wing-wielding Horseman of Apocalypse.
Despondent over the loss of his wings at the hands of Harpoon and the original Marauders in "Mutant Massacre," Angel is taken in by Apocalypse and transformed into his villainous persona. Angel as Death fully debuted during "Fall of the Mutants," in Walter and Louise Simonson's X-Factor #23, wherein he fought against his former teammates while under the influence of Apocalypse.
He was later able to break free of Apocalypse and reconcile his two personas to develop a new identity for himself: Archangel. This story was also adapted on X-Men: The Animated Series and in X-Men: Apocalypse.
Cyclops
Scott Summers, aka Cyclops, was the first mutant Charles Xavier recruited and one of his most loyal pupils. However, when the mutant race was on the brink of extinction, Cyclops welcomed the Phoenix Force to Earth in hopes that the entity would be of benefit to mutantkind, opposing the Avengers in the process.
Cyclops absorbed the power of the Phoenix, attacking his teammate Emma Frost for possession of it. This action was enough to transform Cyclops into the reborn Dark Phoenix and his turn to villainy was complete when he used his power to murder his former mentor, Professor X, in Brian Michael Bendis and Olivier Coipel's Avengers Vs. X-Men #11.
Beast
The first glimpse of a dark version of Hank McCoy, aka the Beast, was seen in Age of Apocalypse. This alternate reality version of the character was a frightening, mad scientist who performed brutal experiments on his own kind with little regard for the pain and trauma inflicted on his test subjects.
The 616 version of Beast has made moves toward becoming more like his Age of Apocalypse counterpart in Dawn of X, so much so that S.H.I.E.L.D. has referred to him as one of the most dangerous mutants alive because of his penchant for doing whatever he pleases without considering the consequences.
This godlike attitude was most recently seen in the pages of the Benjamin Percy penned X-Force, wherein Beast unwittingly created a species of "plant sentinels" while attempting to forcefully lobotomize an anti-mutant activist.
Professor X / Onslaught
Professor X is the mentor and founder of the X-Men, but he has many dark secrets. After Xavier wiped Magneto's mind after the Master of Magnetism ripped the adamantium out of Wolverine, Xavier's dark nature combined with Magneto's to form Onslaught -- a being so powerful that the combined might of the Avengers and the Fantastic Four were required to eventually defeat him.
Professor X recently embarked down another dark path in House of X and Powers of X, wherein the professor leads the X-Men to form their own mutant nation on the island of Krakoa. After inviting several high profile villains to join the nation, Xavier offers them a seat at the governing table and has since engaged in questionable tactics, eliciting the ire of both humans and superhumans alike.