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.
"We did the last one and things ended up fine for us."
They won't need a reason.