> It's not something we even argue over. Murder = Bad. No disagreement across cultures.

This is not true at all. Not even close. Sneaky backstabbing murder by a group member against another group member in violation of implicit group norms has probably always been "bad", but "go out and murder some random human" was a rite of passage for many cultures, raids against other groups for no reason at all except for fun and maybe women were typical across perhaps the majority of groups for thousands of years, and history is full to the brim of wars prosecuted for no particular reason at all.

This goes well into the historical period and there are doubtless groups today still with the same attitude. Why did the Athenians murder the entire male population of Melos despite their neutrality? Because the strong do what we can while the weak suffer what they must.

You are confusing your modern-day HN-poster social norms with some constant of human nature.