Objective morality is like objective tastiness... it just doesn't really make sense.
Obviously for most people tastiness is basically the same: Stuff with (a reasonable amount of) sugar in it: Good. Stuff that's clearly rotten: Bad.
But the argument that sugar is good because of some empirical law of the universe is just strange. No, sugar is tasty because sugar=calories and lack of calories is pretty much the prime way life dies.
Similarly the argument that rotten meat isn't tasty because its just... evil or something is equally strange, no, it tastes bad because eating it will mess you up and possibly kill you.
But if someone went: Nah, rotting meat *is* tasty I would give them a look, because while there is no true objective tastiness, there is objectively standard human tastes, which almost always go "nah fam, don't eat that shit".
Of course this isn't absolute either, tastiness depends on both nature (your tastebuds, DNA) and nurture (what you ate growing up, how that one time when you were 6 you had bad sushi and threw up for two days and thus as an adult hate raw fish). So while most people would look at Lutefisk as being objectively nasty that doesn't mean that it actually is, or that said opinion of rotting stuff=bad even applies to all humans.
---
Its the same with morality, even though say, cannibalism isn't objectively wrong that doesn't mean I won't look at anyone who says its fine as being kind of crazy in the most generous interpretation.
But of course as with many other 'evil' things its trivial to think of circumstances where cannibalism isn't wrong, most notably where the other person is already dead and your only way to survive is to become a cannibal. By the same token its also a thing that many cultures throughout the world partook of willingly and *didn't* think was evil.
I would look way back, to human early history. Their morality was simpler, - what is good for my tribe is good, what is bad for my tribe is bad. Therefore, killing a competing tribe and taking their resources is good. And it comes directly from biological evolution, from the rudimentary morality of non-human social animals.
I disagree, conflict is and has always been extremely expensive (assuming the other group isn't way weaker), and the other tribe can cooperate with and help you as well, which can be very significant when life is as risky as it was back then.
Back at the lab, researchers analyzed them to find that they were black pigments: The oldest paleo-crayons ever discovered, dating back to around 300,000 years ago.
That was only the beginning of the intrigue. Having long studied this site and this period in human evolution, Potts knew that early humans generally sourced their food and materials locally. These “crayons,” however, were clearly imported. They’d formed in a briny lake, but the closest body of water that fit that description was some 18 miles away. That was much farther than most inhabitants likely would’ve traveled on a regular basis, given the uneven terrain. So what was going on?
The pigments, Potts and his co-authors now believe, were part of a prehistoric trade network—one that existed 100,000 years earlier than scientists previously thought.
Even hundreds of thousands of years ago trade was a thing, and along with alliances (which certainly existed as well) there would be incentives to maintain relationships with neighboring tribes and travelers. If you don't care about them at all your members are more likely to start conflicts (which again, are very expensive), so part of ethics, even way back then presumably included caring for people not in your tribe to some small extent.
That said if they are messing with you or otherwise impacting your chance of survival *and* they are too weak to stop you then just killing/enslaving them all becomes the logical choice, so you can't care about them that much.