Charles Xavier and Cassandra Nova

Charles Xavier is often depicted as a benevolent leader of the X-Men, but he has some skeletons in the closet, like his attempt to kill his twin sister in the womb. Charles detected that his sister was evil, and his subsequent attack led his mom to miscarry. Regardless, Cassandra Nova held onto life as a collection of cells and eventually rebuilt her body. As an act of vengeance, she convinced Donald Trask III to send his army of sentinels to the mutant island of Genosha.