[Yoda9864]

God and Satan are in an eternal struggle for the souls of men. Each side coerces people so that their souls may be taken. It's all explained in the movie Constantine.


Of course, this whole question relies on the fact that you believe in some higher power. If you do not, then the answer is simply that bad people exist and there is nothing you can do about it. Good people exist, and bad people exist. They balance each other. If they didn't balance each other (i.e.- one side massively outweighed the other), the world could not function. We would self destruct.