Right, it's a larger number of people but in large part that is because there simply were more people alive at that time. The Thirty Years' War was frightfully destructive, not reaching quite the peak intensity of the Khmer Rouge, but with similar fractions of the population killed.

Anyway, I think we pretty much agree. I just think your original characterization of your position wasn't a particularly fair way to state it, but the elaboration seems fair enough.

You did add another thing--where moral authority comes from--but that has different answers depending on where it nominally comes from and where it actually comes from (proximally, not in origin).

The proximal source of morality seems pretty clearly to be moral intuitions that people have; they can be somewhat sculpted by culture, but only somewhat, at least if extant cultures are any guide.

The ultimate justification is what philosophical ethics is about, and there are all sorts of solutions there, none of which seem entirely satisfying because there tend to either be gaps in the logical reasoning, and thus fail to really justify, or they fail to fully cohere with our moral intuitions (c.f. trolley problem) and thus aren't obviously actually morality instead of some morality-adjacent framework.