10 Comic Heroes Who Killed Their Biggest Rival

A fight that occurs in the literal womb, Charles Xavier and Cassandra Nova's rivalry was one that started as early as a rivalry possibly can. It also made Xavier a murderer literally as early as possible, by having the man's mutant powers manifest while the pair were still in utero, leading to Xavier murdering his twin then and there.

While this is justified in-universe by saying that Xavier 'sensed Cassandra was an evil prescence', it seems kind of brutal to say that about someone who has literally just grown a brain - especially since Charles also had, and thus likely wasn't the best ever judge of character.

Somehow, being born dead didn't stop Cassandra, and she would spend her youth growing on a sewer wall, which isn't the worst childhood ever, but sure isn't the best.