Mandark

Mandark is a prominent antagonist in the cartoon series, Dexter's Laboratory and is in many ways the Anti-Dexter: while Dexter is an incredibly knowledgeable (but often mad) knowledge with an otherwise ordinary life (having to keep his experiments secret from his family) Mandark is an openly fiendish inventor who is not motivated so much out of curiosity as he is greed, hate and (often) outright malevolence.

Mandark takes great joy in inflicting misery on Dexter but due to his obsessive love for DeeDee Mandark has at times allied with Dexter - though these unions are rare and doomed to failure due to the two having very different views on the world (as well as an intense rivalry).

Mandark is arguably seen at his most evil during the one-hour special episode / mini-movie known as "Ego Trip" in which he progressively grows worse in varient futures: to the point of becoming an outright monster by the end of the movie who engages in a "final showdown" with his nemesis, aided by an entire army of variant Mandarks from many different time-lines.