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Here's a cool origin. Mew wanted to be friends with humans. So he morphed to mewtwo to have an aparece that resembles humans thinking that would make contact with humans easier. When he came in contact with humans he realized he was never treated like an equal but as an inferior animal. Humans quickly started to conduct experiments on him to find how his psychic powers work thinking this will be the key to understanding every Pokémon power and ability (because he can talk and other Pokémon are stupid). Realizing that mew would never be treated as an equal leaves humans to go in eternal hiding again. But a part of him still felt frustrated and angry. That part separated from mew to become a new entity that hates humans. That's mewtwo.
Yeah I copied Majin Buu.