So far historically it was "they're not Christians", then it was "wrong location" (cause they wanted the land/resources), then it was "they're not the right race", then it was "wrong economic system", and now it's "wrong culture"; what's next do you think?
EDIT: By justifiable I don't mean it's actually justifiable, I mean most people will consider it justifiable.
It's really always just us vs. them. That principle encapsulates all the reasons you listed.
Our benevolent humanitarian intervention vs. their unprovoked, brutal war of aggression. It's really just the meme.
We always mean good, they are pure evil, so we had to kill them all. We didn't intend to kill all those women and children, it's their fault.
What's amazing is how many people actually, really believe that, even with college degrees and all that.