Mysterio

Mysterio might actually hold the title for the man who’s caused the X-Men the most pain, based mostly on his appearance in Old Man Logan. In one of the most brutal and horrifying moments in comics history, Wolverine wakes up one day to find the X-Men mansion overrun with enemies. He kills scores of supervillains before Mysterio reveals that it had all been an illusion—Wolverine had in fact been killing all of his fellow X-Men, as well as the mansion’s young students, as they begged for him to stop. Afterward, Wolverine left the mansion and didn’t unsheathe his claws for 50 years, living as a shell of his former self.