The Evil Doppelgänger is traditionally a Mirror Universe counterpart. The double can also be the product of a What If? scenario: 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. (Although sometimes a clone may have a case of Copied the Morals, Too.)
