Meldar Prime

Meldar Prime is the villain of the Jimmy Neutron movie episode "Win, Lose and Kaboom".

He consists of an organic head attached to a robotic body and is the host of a game show called "Intergalactic Showdown" in which aliens from different planets have to compete against each other in several challenges, and all but the winners have their home planets blown up. Meldar does all this using his matrix generators, which give him virtually limitless reality-altering powers and prevent inhabitants of the galaxy from stopping him.

He casually treats this mass genocide, which he seems to have no motive for other than entertaining himself, like an ordinary game show, making jokes out of blowing up planets, and April says that he has been doing so for "eons". Not only is Meldar without question the most evil villain by far in the Jimmy Neutron series, but one of the most prolific mass murderers in all of fiction. At the end of the episode, Jimmy and friends manage to steal Meldar's matrix generators and destroy them, ending his show once and for all and leaving him to be tortured forever by the Brains (one of the other races they were competing with on the show).