I think this makes the common mistake that all "primitive" cultures have the exact same behavior -- I suppose you're not speaking from PNG right? (although likewise I have no such specific knowledge) As the article cites, in this specific place (and in many other similar cultures), I don't think there's motive to wage large scale wars or take large amounts of territory, that would justify something like raiding and wiping out other groups. I suppose that could happen in the huge number of other cultures, ones that had more centralized leadership or competing more unified cultures. The conflicts in the case of PNG might have been localized dispute over borders and such.

In nonhuman animals, even in fierce disputes over territory and mating, killing is rare, because it's quite costly: you eliminate completely a healthy individual, and usually would run the risk of serious injury yourself, making both dying likely. Usually they e.g. resolve mating with displays of beauty/fitness, and territory with displays of force (chimpanzees for example have lots of display of force rituals, specially around selecting leadership, I believe) and sometimes non-lethal conflict (some species evolve apparatus just for this kind of conflict, e.g. antlers, so they can engage in conflict with low chance of injury).

Since we're referring to human cultures, something distinct we have is memory and communication -- information can get out on reputation, behavior, etc. of a group. I think if a group just goes out and murders another in their sleep (which TBF is not so easy), then I imagine for example word would also go out of their actions and may get into trouble with other groups, I'm sure spiritual traditions would also have a say about it, in favor of some sort of "spiritual revenge". But I suppose it's difficult to generalize.

I do think it's interesting to consider some forms of morality, or just metaphysical and logical understanding around ethics, as a kind of technology that enables people to live longer and enable more secure conditions, including preventing large scale conflict that can bring destruction on an industrial scale to entire societies (although misguided moral arguments ironically are also misused sometimes to enact destruction). Although I guess morality alone is not enough (because not everyone seems to internalize it well), and other mechanisms are also helpful to achieve more social stability.

(Also of note is that there can be such thing as too much stability -- for example, for some new more interesting and maybe more worthwhile forms of culture to emerge, old ones have to be replaced, so stability should be balanced with cautious openness to new ideas, technologies, methods, peoples (ethnicity), cultures, ..., and worldviews -- of course, as long as change is in the direction of improving our lives collectively. For example in some cases religious fundamentalism advocates for an absolutist cultural stagnation; we just shouldn't need one group to murder another to spread about new things)