I'm going to assume that only the rules the OP mentioned apply because no possibility of an afterlife just kills the thread.
I would slowly bump off everyone in the world I don't like, for instance: Justin Bieber, Rebecca Black, a few jerks from high school, Osama just to be sure, Ray William Johnson and various celebrities I never want to hear from again (Jennifer Garner, Angelina Jolie, the chick from Twilight etc).
I wouldn't even bother with the whole "change the world by killing all the evildoers" thing because I'd end up killing maybe 1 or 2 guys a month. Seriously, try finding the names and photos of large amounts of people who've committed serious crimes. It's really hard.