It's not doubt that two of the strongest mutants are Professor X and Magneto. Mutants are graded by levels, I believe, and these two are level fives. Now imagine if you combined them. Now imagine even further. Not only did you combine them. You combined the darkest parts of both of them. That, ladies and gentlemen, is Onslaught. He looks more like Magneto than anyone else, but I bet once you take off that helmet he's bald just like Professor X.
His plans was to destroy everybody, humans and mutants alike. Deep down, that's probably what both Professor X and Magneto knew was right. It's like when your parents would turn the car around because you and your sibling were fighting. "Screw the whole thing! It's over!" Luckily the X-Men gathered with some other heroes, including Thor, and killed him.