What Happened To Everyone Who Used Cerebro?

Cassandra Nova is the only character on this list who can be considered truly evil. She's a parasitic lifeform called a mummudrai who is telepathically entangled with Charles Xavier and even found a way to psychically create her own body so she could exit the womb and be born.

Clearly, she is a powerful telepathic being, but the scope of her powers is far greater than that of Xavier. She can perform acts of telepathy, telekinesis, become intangible, shapeshift, is immune to magic, can see future events, perform mind control, and has numerous other invasive telepathic abilities.

Her biggest claim to fame was an actual genocide in which she commanded an army of Sentinels on an attack against Genosha. That resulted in the deaths of some 16 million Mutants, which makes her pretty much the polar opposite to Charles Xavier.

Speaking of Xavier, when Cassandra opted to use his favorite device for her own purposes, she managed to swap bodies with him. This was after the system was upgraded into what became Cerebra, but she managed to use it to exchange her mind with Xavier's, and there's not much more invasive and creepy than that.