In 'X-men', Professor X is entirely morally culpable for all of the death, destruction, and antagonism towards mutants wrought by Magneto, because again and again Professor X fails to kill Magneto when he has his seemingly limitless set of chances. 

The fact that Professor X seems to do so out of a combination of loyalty to his own kind (mutants) and personal friendship with Magneto makes his inaction even more damning. Professor X even prevents others from killing Magneto, some of whom almost immediately later become Magneto's victims.

You can (and people often do) have the moral imperative to kill, regardless of what you have been taught to believe since you came out of the womb.

I know this probably isn't news to many people's worldview. But from my experience, it is something you are not sufficiently actively considering. It's convenient to ignore, so I charge that you probably are.