Putting aside the character's bizarre (no pun intended) origin story which revealed that he used the duplicating ray which led to his creation to make "Bizarro World", this Superman clone who speaks in opposites has been a major problem for the Man of Steel for decades now. Portrayed both as fiercely intelligent and hilariously stupid, any version of Bizarro is a threat to Superman because they obviously share the same powers. Well, sort of. Bizarro has freeze vision instead of heat vision for example, but the point is that he's capable of causing the mass destruction Superman would never dream of doing. His mind is in fact so twisted that he destroyed Bizarro World on purpose - killing the billions who lived there - simply because Superman's home of Krypton was destroyed by accident. He may talk funny, but Bizarro is a very serious threat.
