WARNING: The following contains major spoilers for House of X #5, by Jonathan Hickman, Pepe Larraz, Marte Gracia, VC's Clayton Cowles and Tom Muller, on sale now.
A big part of the reason that writer Jonathan Hickman's House of X and Powers of X series are successfully reinvigorating the world of X-Men comics is the mystery behind what all of this is ultimately leading. From the get-go, the titles have seen Xavier established a new nation on Krakoa to unite the world's mutants and asserting themselves as a unified front against threats like the creation of Nimrod.
But now, House of X #5 has dropped a major indication that this story might end with Professor X becoming Marvel's next major villain.
As always with X-Men comics, the line between hero and villain is hazy and the grey morality in the story is a playground for discussion over what is and isn't justified. Up until House of X #5 the position of Xavier and his team of mutants seemed wholly defensible. After suffering decades of persecution and even attempted genocides, the X-Men are drawing their line in the sand.
After the apparent death of main characters like Wolverine and Nightcrawler in the previous issue, House of X #5 opens with the reveal that Xavier has apparently had a contingency plan in place before sending them to their deaths. By combining the powers of five different mutants at his disposal with his own telepathic powers and genetic material from Sinister, Xavier was able to perfectly replicate clones of the heroes lost.
Xavier explains that he uses Cerebro to back up every mutant mind on the planet on a weekly basis, forming a database of mutant consciousness he can upload into a new body in the event of their death. The infographics Hickman provides hint at further, potentially more sinister applications of the cloning technique, such as cloning multiple copies of an individual or uploading a consciousness into a body different from its original.
Playing God and outright defying death would not inherently be indicators that Xavier has turned to the dark side, but the issue further presses the point by showing Xavier's willingness to compromise his ethics. At a U.N. summit to formally grant the mutant nation legal recognition of their statehood, Xavier not only allows another telepath, Emma Frost, to use her powers to influence the will of the diplomats deciding the vote but actually condones it.
The end of this issue details Xavier successfully inviting many of the biggest villains in X-Men history to his cause. Mr. Sinister, Selene, and even Apocalypse himself all step forward to join forces with Xavier. Many of these villains are responsible for hundreds or thousands of deaths in their own right, but Xavier seems completely comfortable with utilizing their deadly means toward his own ends.
As Xavier showed Doug Ramsey around Krakoa in Powers of X #4, he wore a safari-inspired outfit that was almost identical to his evil twin sister Cassandra Nova's most famous look. Cassandra was noticeably absent from the massive group of villains, which included well-known threats and obscure villains like Forearm and Random, who joined Krakoa in House of X #5. Considering her history of impersonating Xavier, it's possible that her influence could be somehow at work here too. However, Nova was notoriously anti-mutant, however, and her ability to mask such hatred for the cause of mutant unity would seem more questionable.
Whether there is another villain's influence at work or not, it's clear that Xavier is being established as a major level threat. Given his immense power, and the further power now cultivated toward his cause with so many allies, he could very potentially be one of the most dangerous threats to humanity that the Marvel Universe has ever known. The fact that he has had such an easy time gathering such a diverse range of allies, underpinned by his clear willingness to influence the minds of others, could indicate that he is already at work using his powers for evil.
Xavier has assembled an incredibly wide and diverse cast of heroes and villains, uniting the motivations of characters with practically nothing in common. Even non-X-Men-related mutants like Mentallo (who is mostly a petty crook) and Gorgon (whose only "cause" is generally just killing as many people as possible) wordlessly answered Xavier's call. The fact that Xavier seems to constantly be wearing a Cerebro unit to enhance his powers would justify the possibility that everything that has happened so far has done so according to his design.
However, Xavier almost certainly isn't acting alone in all of this. While the series made a big point early on of establishing the reincarnated Moira MacTaggart as a major mover and shaker, very little has been seen of the newly-revealed mutant since she first approached Xavier about an alliance in the past. The combination of her knowledge of potential futures combined with Xavier's vast telepathic powers could prove to be a deadly combination. If those two are truly united in an evil cause, it's hard to imagine what could possibly stop them.