I think we agree, more or less. Now, to add a corollary to this discussion, consider that existential threats are usually perceived, if not necessarily as sub-human, as non-human. Because somebody who doesn't just disagree with you on some issues but wants to destroy you must be necessarily lacking some fundamental human quality.

So once you've set a narrative of the "it's us or them" type, it's easy to place the blame of your necessary defensive action on the enemy: because the enemy has already renounced its humanity when it set itself on enslaving or destroying you.