The backup in Classic X-Men #19 gives us our answer. The problem is: it's not very good. He was hunting Nazis on behalf of Mossad, only he hunts the wrong Nazis and the CIA get mad at him. They murder his
loverdoctor Isabelle (note: the difference between Charles and Erik is that Xavier has sex with his patients but Magneto seduces his doctor), right in front of him, and then are surprised that he he's a little bit unhappy with that. He proclaims himself homo superior etc.
As I say, this doesn't work. It's a third personal tragedy for Magneto, but it pales in comparison to the other ones. Either he needs to go down that path through simple logic, or something with emotional resonance needs to happen to him. And fridging someone we've never met before just is not enough, not given his previous history. X-Men: First Class got this dead on.