From Carl Sagan’s “Cosmos” (p. 283):
“We have held the peculiar notion that a person or society that is a little different from us, whoever we are, is somehow strange or bizarre, to be distrusted or loathed. Think of the negative connotations of words like ‘alien’ or ‘outlandish.’ And yet the monuments and cultures of each of our civilizations merely represent different ways of being human. An extraterrestrial visitor, looking at the differences among human beings and their societies, would find those differences trivial compared to the similarities. The Cosmos may be densely populated with intelligent beings. But the Darwinian lesson is clear: There will be no humans elsewhere. Only here. Only on this small planet. We are a rare as well as endangered species. Every one of us is, in the cosmic perspective, precious. If a human disagrees with you, let him live. In a hundred billion galaxies, you will not find another.”
It is human to disagree, but also human to cooperate. None of those great ‘achievements of human civilisation’ could exist without cooperation. Many would also not exist without some form of slavery. We should be less concerned with the opinion of a theoretical extra-terrestrial visitor (who might be better or worse than human) than in putting our human house in an order that makes common sense.