10 Comic Book Characters Who Are Just TOO POWERFUL!
4. The Joker (as Emperor Joker)
During the Emperor Joker storyline, Joker tricked inter-dimensional imp Mr. Mxyzptlk into giving him 99.99% of his power. Anyone who knows anything about comics can figure out what happened next.
Within seconds of being omniscient and omnipotent, Joker had completely recreated reality in his own twisted vision. Everyone else in existence except for him was caught in an endless timeloop, which Joker took full advantage of.
He killed Lex Luthor repeatedly. He tortured and killed Batman every day, then revived him overnight to repeat the cycle. He consumed the entire population of China and beat Jimmy Olsen to death with rubber chickens.
In other words, he did some really bad stuff.
Emperor Joker was only defeated when Superman forced him to realize that despite his power, he couldn't kill Batman, because without Batman he wouldn't exist. This caused the Joker to lose control of his power for a moment, allowing Mxyzptlk to take it back.
This version of the Joker literally used the universe as his plaything, slaughtering billions while laughing and making jokes all the while. It was only a combination of Superman's will and the Joker's ego that let him be defeated; otherwise he may still be running loose around the cosmos.