There are no fantasy worlds. There are no orcs. They don't exist. All that exists is a bunch of humans writing stories to each other about how cool it would be if we could finally let loose and stab some folks that looked different without having to worry about boring stuff like their inalienable rights. I happen to think that maybe we should be a bit better than that.
If nothing else, it would increase the overall quality of the genre's storytelling if every antagonist needed to actually have a reason for deserving death.
All that being said, this discussion smacks of a "Morally Justified" thread, with multiple dips into real-world analogies. Thread locked.