Evil Doppelganger

The Evil Doppelgänger is traditionally a Mirror Universe counterpart. The double can also be the equivalent of For Want of a Nail: he/they represents what the hero or heroes could become, or may become.

Sometimes the evil double is a clone and may brood about how he's not a "real" person, just a hollow imitation of the original. If that happens, the clone tends to go mad and try to murder the original to take his place.

The evil doppelgänger is not to be confused with the original doppelgänger. This character is more like the Evil Twin. See also Criminal Doppelganger and Evil Counterpart.

Anime & Manga

 * The Copy Rockman/Copy Robot from Mega Man Megamix is a good example. He's a copy of Mega Man that believes himself to be the real one and has his memories, but then he goes and tries to kill Dr. Wily. This helps to prove that he's not the original, due to the whole Thou Shall Not Kill thing. He's more powerful than the original, but has a defect that can overload and destroy him, guess what happens in the end. When Copy Rockman finds out he's a copy created by Wily,.
 * Sonic the Hedgehog The Movie had Metal Sonic act like this, that is until.
 * Magic Knight Rayearth had Nova.
 * Mewtwo from Pokémon the First Movie franchise may count. He does eventually do a Heel Face Turn of sorts.

Comic Books

 * Bizarro is an imperfect clone of Superman, but Bizarro may be considered as more of a victim himself as he's lacking in the intelligence department.
 * Superman's actually evil counterpart is his Mirror Universe double, Ultraman (no relation).

Film

 * Star Wars has Emperor Palpatine as the Evil Counterpart to Master Yoda.
 * Godzilla has two: Mecha Godzilla and Space Godzilla.
 * In Toy Story 3, Woody meets his not-so-nice Doppelgänger in.

Live Action TV

 * Star Trek, of course, has the Mirror Universe, with evil beards and everything.

Web Comics

 * In The Inexplicable Adventures of Bob, Galatea is an emotionally unstable, sometimes dangerous clone of Molly, but it would be unfair to call her evil. "Chaotic" though? Sure.