This is simply not true, in time of severe distress and survival pressure humans are clearly capable of mass killings. It happened so many times throughout history. For example a famine forces a human group to take over rivals resources or when defending own group against agressive rivals.
Of course it's true, and that there are 'acute moments of distress' doesn't disprove the claim.
'Murder' is nearly a universally negative social concept.
There zero cultures wherein arbitrary killing is considered acceptable
Nobody ever said that these murders are arbitrary. They're the opposite of arbitrary. They're coalition-based murders against men in the opposite tribe. Highly targeted and intentional. Not random.