Mewtwo truly has the most tragic backstory. Cloned from the Mythical Pokémon Mew, this creature was born in a lab. Cursed with great intelligence and Psychic powers, it was unable to do anything but watch as its other clone friends died and withered away in the lab with it.
Once it broke out, it promised to never allow itself, or any other Pokémon, to be imprisoned in such a cruel manner ever again. This translates to being a Pokémon badass.