I agree with some of the posts here:
Per capita w/ respect to population as a whole - Pol Pot
In general - Stalin
Greatest quantity intended (if plans fully completed) - Hitler
Mao is right up there with Stalin, and I've heard of the 11-year old girl reference before, I believe in some book I read in college. I couldn't tell what the title was though. There is (or was) a belief in China that virgins would revitalize the soul, center your chi, that kind of thing - and therefore were to be prized by those in power, especially Mao...I think you can see where I'm going with this, so I won't continue. Wasn't just 11 year olds, either...
Though I can't cite a specific reference more than having read it at some point.
My grandmother (who lived through WW2) would tell us stories that essentially spoke to the inhumanity of the invading Japanese, about hiding to escape from Japanese soldiers for fear of their lives. There wasn't any *one* soldier, but so many of them participating in slaughter both in China and other areas where they conquered that spread out the mass killings.
As far as Hussein and Milosevic are concerned - didn't they share a similar nickname - the Butcher of Baghdad and the Butcher of Belgrade?