evil

Night and Day.
Jedi and Sith.
Gods and Demons.
Yin and Yang.

You can't have one with out the other. Life seek balance. For every one extreme there is it's equal on the other end of the spectrum.
There is a purpose for its being.
Weather it be a creation of environment, a swapping personality inhibitors, a determination to change things, the enjoyment of maniacal laughter, an understanding of things others don't, vengeance, pride, or the willingness to do something.
Explanations can be far and wide, complex descriptions of being to the world. Or as short and direct, simple as a blatant statement to your bumbling superhero brothers.