Evil is that which effects changes. It does so using methods that we can not understand or rationalize; or worse does so by a manners that we simply can not accept as the correct path by our own definition. Evil is the force that does things that we wish never happened in the first place.
Granted this definition almost suggests that evil is powered and fueled by our own limited understanding and comprehension.
I remember reading somewhere once that the Gods got their powers from the belief and acceptance of their worshippers. Thus the old Gods of Greece exist but are weakened by the fact that most people in Greek are now Greek Orthodox and follow the Christian deities.
It amuses me, that we could power evil just as much as any other personification of a concept with a combination of belief and naivety.
What is evil to you?
Edit: I always get affect/effect backwards. This has been fixed. Thank you to tiggr93 and jnanacandra