User blog comment:Drakan95/Kenny vs Deadpool. Epic Rap Battles: Cartoons vs History. Season 2/@comment-34129479-20180525225325/@comment-34129479-20180526054602

In Captain Underpants, Principal Krupp is hypnotized by his troublemaker students, George and Harold, into acting like their made up superhero, Captain Underpants, when someone snaps their fingers. Krupp is a complete jerk who takes joy in punishing the two boys, whereas his Captain Underpants (his alter ego whom he's not aware of) is kind and heroic (albeit, kind of a dumbass).

SPOILERS FOR FIGHT CLUB INCOMING: In Fight Club, the unnamed main character, played by Edward Norton, finds a companion in Tyler Durden, and the two start the titular "Fight Club", which transforms into Project Mayhem and quickly goes into full-blown terrorist activities. It is during that turn of events that our main character finds out that Tyler Durden isn't real, and is, in fact, just an alternate (ableit morally black) version of himself.

TLDR; They're both people with split personalities. One is heroic while the other is a bordeline terrorist.