My understanding is that it's a settled question and that the evidence of extremely high homicide rates in hunter gatherer societies is overwhelming. If you have any links suggesting otherwise, I'd be interested in seeing them.

As for relatedness, that's relative (no pun intended). If everyone within 200 square kilometres is at a genetic distance of a second or third cousin, then a second cousin is the base for a non-relative. In any case, regardless of why precisely it is that violent death is so common pre-modern and especially pre-state societies, it's abundantly clear that it is.

Regarding low population densities, you'll notice that Australian Aborigine tribes are represented in that chart and exhibit murder rates that are far higher than those in modern societies. They do however have the lowest homicide rates among hunter gatherer tribes. The highest homicide rates are found in the Amazon rainforest, where population densities would be highest. So it appears that the two factors that contribute to high homicide rates are pre-state/agricultural society and high population density, and that in the case of the tribes living in the most sparsely populated regions, the inhibiting effect of extremely low population densities on homicide rates does not make up for aggravating effect of living in pre-state societies.