taw:

For another example - killing own babies was extremely widespread, not even condemned in any way in most cultures including pre-Christian Rome. It was just as casual as abortion is today.

Or killing family members who disgraced your family in any way (your judgment) is widely praised in many cultures.

You're right about this. Attitudes towards infanticide vary greatly between cultures, and in many cultures, both past and present, the recognized authority of the senior family/clan members has included the power to enforce their will, and the standards of behavior, by threats of death against the subordinate family members.

But again, all this always happens within a legal structure with clear rules about what constitutes unlawful killing, and serious penalties for those who kill unlawfully. This legal structure may have the form of unwritten folk custom, but people living under it are no more capable of ignoring it than the citizens of modern states can ignore the codified criminal laws. (In fact, even less so, since many laws nowadays are enforced only sporadically or not at all, and flouted widely and openly.)

You might be confused by historical record, as cultures without centralized states and legal systems it brings tends to lack writing as well - so most of our records come from highly unusual subset of cultures with centrally enforced law, relatively individualistic societies etc.

You don't have to reach for misty prehistory, or even for particularly exotic and remote parts of the world, to find examples of traditional societies where the formal state-enforced law is largely irrelevant. For example, there are still ongoing clan blood feuds in some places in the Balkans, specifically in parts of Albania. These examples show a huge problem with informal revenge-based folk justice, namely that vengeance for individual killings can easily escalate into out of control clan warfare. Yet all this only goes to show how serious a transgression it is.

There was no "murder", no "guilt", and no "penalty", and no "legal reparation" (in narrow modern sense) anywhere here. These concepts make no sense in such cultures. Intentional killing is treated identically to a common accident.

Your disagreement is with the terminology used in your own cite. But the terminology doesn't matter. If your cite is anywhere in the neighborhood of accurate, then these cultures held that something wrong had happened — that is, some imbalance had occurred that had to be righted — when one person caused the death of another. Perhaps they considered intentional killing to be no worse than a common accident. But another way to say that is that they considered unintentional killing to be just as bad as intentional killing.

You might be confused by historical record, as cultures without centralized states and legal systems it brings tends to lack writing as well - so most of our records come from highly unusual subset of cultures with centrally enforced law, relatively individualistic societies etc.

If the cultures you want to use for your argument left sparser evidence, then that means that you should be less confident about your interpretation of the moral thinking that was behind the written laws that they did leave behind.

In fact, you have a greater burden of proof than your interlocutors. You are saying that a culture that left less evidence was different in a particular way from most of the cultures for which we have better evidence, whereas your interlocutors are saying that the less-evidenced culture was probably about the same in this particular way. That means that your hypothesis is more complicated, and so a priori less likely.