Let's say when you do wrong X to someone, it's quite possible that they return the "favor" someday. I don't know what we are arguing here. Are they evil? For sure, but do you really expect them to be gentlemen revolutionaries who only kill those "should be killed"?

Actually Europeans did the same in not so long ago, not as slaves, but as slavers (or some more fancy words), so people can go down much further IMHO.