What would be the worst thing to happen to the Marvel Universe? Well, probably the death of all of its greatest heroes, like Captain America, Thor, Iron Man, and Hulk. Guess what? That happened once, and it was all the X-Men's fault. During a fight on an asteroid in Earth's orbit, Magneto decides he's had enough of Wolverine and rips his the metal coating his skeleton out of his body. This is too much for Professor X, who goes into Magneto's brain and shuts it down. Wait, Professor X can do that? How many genocidal maniacs live in the Marvel Universe that Xavier could just stop with a thought? Anyway, when Xavier does this, some evil from Magneto's mind jumps into Xavier's brain, the two combined, and the being known as Onslaught is created. When he shows up in the comics, Onslaught is super powerful and plans to destroy the Earth or kill all the humans…or something. He basically has a generically evil villain plan that's never really fleshed out. During the fight, all of the aforementioned heroes show up, and the whole thing ends with their deaths. Technically, Professor X is responsible. Luckily, all of the heroes are returned to life about a year later because Marvel Comics realized that killing off half of your most important characters is a bad idea. Still, Xavier most likely killed your favorite Marvel superhero. After sending a team of kids to die on a living island. And after he had impure thoughts about one of his teenage students. Professor X is gross, you guys.