how do we know that "Killing someone for disagreeing with you was a perfectly acceptable option in all societies"?
Not many where it wasn't, really. Even in Europe, formal duels were commonly fought until the early 19th century, and it persisted as a minor option for longer than that. Germany kept the Mensur until well into the 20th century, although admittedly that wasn't intended to be a duel to the death.
Does that mean that death did not bring grief?
These days, people say they'll never get over their son/daughter/partner dying. It's often said that no-one should out-live their children. And these days, I think that's a good response - it indicates how much you value those people close to you. But if you were a parent back then, you'd *know* that you're going to have something like 15 children if you live long enough to die of old age, of which probably 3 will die in the first year, another 5 maybe before they reach 16, and another 2 in their 20s and another 3 in their 30s from various diseases, agricultural/industrial accidents or military service. After the first 2-3 kids as a woman, you're probably not going to die in childbirth, but you're going to see most of your kids die before you do. As a man, if you live to 50 then you might have had a half-dozen wives over the course of your life, as previous ones died in childbirth.
The only comparable situation today to parenting in pre-medical days is with medical staff, who know that some proportion of their patients will die. They can get attached to them and be sad/upset when they don't make it, but it's something they get used to. If you imagine an entire society like that, where death of friends and close relatives is so commonplace and something you learn how to deal with in early childhood, then I think they simply *couldn't* have felt the same kind of grief over a death as we do today - or the ones who did would have been exceptions to the rule.
As you say Ebbie, we can't know for sure what anyone felt back then. All we can do is find comparable situations in modern day niches, look at people's reactions to them, and extrapolate it to an entire society filled with people with that attitude. As far as death goes, the medical profession is the only example these days - even soldiers don't have the kind of death-toll that a newborn baby had back then. The one constant in humankind seems to be that people learn to deal with whatever situation they're in by adapting their mindset so that whatever unpleasant thing doesn't bother them as much. In this case, the unpleasant thing is death of close friends and relatives. When that stops bothering you - well, how can death of other people you don't know matter?