The WTF Backstory:
Except that Mario is the villain in Donkey Kong.
According to the game's manual, Donkey Kong was actually Mario's pet ape. Without even venturing into the shaky moral and legal issues of primate ownership, it gets way worse from there: The reason Donkey Kong escaped in the first place was only because Mario was abusing him. That's not our accusation; the manual spells that out, plain as day: "[Donkey Kong] is actually [Mario's] pet who was mistreated." The manual doesn't get really specific as to exactly how DK was being abused -- presumably because even jaded 1980s game designers figured that was some heavy s**t to lay on a kid -- but it's not hard to fill in the blanks: Here's a screengrab of Mario sticking Donkey Kong in a cage, chaining him up, and what's that in his hand? A whip?
Isn't this basically how the Planet of the Apes got started?