10 Most Evil X-Men Villains
2. Cassandra Nova
The X-Men comics have always prided themselves on their soap opera elements so of course Professor X has an evil twin.
This being comics though, said evil twin is actually an alien parasite who used Charles' DNA to make herself a body in his mother's womb. Cassandra's thoughts were so evil that baby mutant telepathic Xavier strangled her. When even a fetus thinks you're too evil to be allowed to live, that's maybe a bad sign.
Charles would be proven right years later, when Cassandra Nova rebuilt herself and introduced herself to the X-Men with a bang. Taking control of the Sentinels, she set the mutant-hunting robots on the island of Genosha, wiping out millions of inhabitants.
And that was just the start of her plan. Cassandra allowed herself to be captured by the X-Men so that she could be brought back to their mansion. Once there, she swapped her mind with Xavier's and got to work. She psychologically crippled most of the team, mind-controlled a student into beating Beast into a coma and then drove Charles' lover, the Empress Lilandra of the Shi'ar alien race, insane and had her attack her own planet.
Her rampage was only stopped by the combined efforts of Professor X, Jean Grey and Emma Frost, who were able to trap Cassandra's mind inside a dead alien shapeshifter.