The X-Men have many different iterations throught the many worlds of the Marvel Universe, and one of their lesser known incarnations existed in the futuristic world of Marvel 2099. This reality is most associated with a futuristic equivalent to Spider-Man, but it was home to future versions of several Marvel heroes, including the X-Men.
In the future of 2099, mutants are still a hated minority, which led to a new team of X-Men to rise and defend the species. The new X-Men were inspired by a man named X'ian Chi Xan, but he would inevitably lead them on a path far removed from the teachings of Charles Xavier. Here's how the greatest ally of the future X-Men eventually became one of their greatest enemies.
X-Men 2099
Created by John Francis Moore and Ron Lim in 1993's X-Men 2099 #1, X'ian, who was also known by the name of Desert Ghost, was a mutant with an incredibly deadly ability. His left hand had a destructive power that disintegrated anything that it touched. He would turn away from his former criminal past, having learned of the X-Men in the past and how Charles Xavier formed the group to change the image of mutants and heal mutant/human relations. He envisioned a new, much brighter future for mutants, and his ensuing actions were hoped to bring this to pass. This was more important than ever, as in the interim of the disappearance of the previous X-Men, human society hard purged and executed numerous mutants in an event called the Great Purge.
A former friend would betray X'ian, leading to his being mortally wounded. Strangely enough, X'ian's body would surround itself in a cocoon, and after coming out of it, X'ian would be completely healed and have the new mutant power of healing people with his right hand. From there, he would lead his team of new X-Men, which included future mutants like Skullfire, Meanstreak and Cerebra, in several missions against those that would oppress them or stand in the way of their mission of peace.
One of their biggest threats was the Chosen, a team of mutants led by the psychic mutant Master Zhao. Arguably having more superficially in common with Xavier than X'ian, Zhao's Chosen were likewise mutants whose powers (optic blasts, psychic abilities, wings, cryokinesis and Beast's animal-like abilities) made them corrupted versions of the original X-Men. X'ian would defeat the now comatose Zhao after a grueling psychic battle, but the event left him a heavily changed man.
Fall of the X-Men
The battle with Zhao made X'ian reconsider his former ambitions, seeing the revival of the X-Men and Xavier's dream of coexistence as failures. His goals now aligned more with Zhao's mutant supremacy, and he would desert his former team to unleash dark vengeance upon those who had struck against him in the past.
X'ian would begin working for the Theatre of Pain, who literally sold pain and anguish to those who so desired to pay for it. This would see X'ian briefly become Controller X, who attempted to inflict worldwide pain through a powerful psychic broadcast. He was stopped by psychic vampire mutant La Lunatica, who made him experience his own pain again and become remorseful at his current path. He became a healer, using his mutant power to help those affected by the Theatre of Pain, though he no longer felt worthy of leading the X-Men. He would still occasionally help them, however, such as when the Atlanteans flooded the planet. This would leave the X-Men some of the world's somewhat scant survivors, building a new society in the Savage Land in the aftermath of near extinction. X'ian was one of their number, and amid his many mistakes as the X-Men's former leader, he taught them perhaps the lesson that Xavier and the other mutants had exemplified before: survival.