This article contains spoilers for Legion of X #3.
Marvel has finally revealed the real reason Juggernaut hates his step-brother, the X-Men's Professor X. The sibling rivalry between Cain Marko and Charles Xavier is the stuff of legend. One is numbered among Earth's most powerful telepaths, while the other has claimed the unstoppable power of Cyttorak, a God of Destruction, and become his avatar on Earth.
The relationship between Juggernaut and Professor X has always been a strained one, tied to some sort of secret mutant war Professor X's parents participated in when they worked at a facility at Alamogordo, New Mexico. It only worsened after Cain Marko became the Juggernaut, because he immediately dedicated himself to destroying his step-brother. He became one of the X-Men's first, and most dangerous, enemies — launching attack after attack on Xavier's School for Gifted Youngsters.
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Legion of X #3, by Si Spurrier and Jan Bazaldua, subtly reveals the real reason Juggernaut hates Professor X. Like so many bullies, Juggernaut is afraid of feeling weak; although his body is weak, he's well aware of the vulnerability of his mind. Presumably this is why Cyttorak granted Juggernaut a helmet to nullify psychic attack while he is wearing it — because he needed the reassurance he would be protected from Professor X's powers. This deep-rooted fear would perfectly explain why Juggernaut has never been able to resist returning to challenge his step-brother time and again.
Professor X has given Juggernaut good reason to fear him. So many battles between Juggernaut and the X-Men have culminated in the mutants knocking his helmet off, leaving Juggernaut
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