The Juggernaut is one of the most famous (and most deadly) of the X-Men's classic foes, and harbors a deep resentment toward the mutant team - but especially toward Charles Xavier, his half-brother. Cain Marko's super-strength and nigh-invincibility don't actually originate from a mutant gene, as the character is not a mutant. He is nevertheless a villain who mustn't be underestimated in any shape or form.
When Charles Xavier's father was killed during an atomic test (Xavier later wondered if mutant powers came from atomic radiation exposure), the power-hungry Kurt Marko married Xavier's mother, thus controlling Charles' inheritance. Cain, Kurt's son from a previous marriage, immediately began bullying the young Xavier, resenting him for his intellect. Cain accidentally killed his father during an argument in a lab, leaving him without a father - and any guidance that might have staved off his bad behavior. When Xavier used his burgeoning mutant mental abilities to foresee Marko's moves in a fight - and thus beat an opponent bigger than himself - Marko loathed Xavier more than ever before.
The two would eventually serve in the Korean War (this was at a time when Xavier still had the use of his legs). Cain Marko, for all of his aggressive bluster, deserted under fire and ran away to a nearby cave. Xavier followed him - reminding him such an offense would grant him a court-martial - and the duo stumbled upon, according to Xavier, "...the sacred lost temple of Cyttorak! Legends have warned of it for centuries!" Cain impulsively grabs a ruby next to a golden statue, reading the inscription: "Henceforth, you who read these words shall become forevermore a human juggernaut!" As Cain transforms, Xavier escapes the cavern just as it begins to cave in, burying Marko.
When Cain Marko - now the Juggernaut - finally emerged from the cave and stormed Xavier's mansion, the X-Men worked as a team to take him down (his helmet protected against Xavier's psychic attacks). He blamed Xavier for his fate, and his hatred spread to the X-Men. Marko would eventually make peace with the X-Men after the demon Cyttorak took a portion of his power, leaving Marko lost and directionless in life. But to stand a chance in a fight against the Incredible Hulk, Juggernaut made a deal with Cyttorak to regain his power - only if he gave in to his selfish ways once more. Hulk won the fight, leaving a bitter Cain believing his sacrifice was in vain.
Cain Marko eventually performed heroic acts, but Xavier does not forgive him for his transgressions. He's notably not welcomed on the island of Krakoa in Jonathan Hickman's 2018 X-Men reboot. The Juggernaut remains an antagonist of the X-Men to this day - if not necessarily out-and-out evil, then certainly one that's filled with anger at his predicament.