When one reads the ancient Greek epics - or in my lazy case, reads about them with some selected passages being highlighted - one sees that slavery was regarded as a terrible thing to happen, but not necessarily a generalised moral evil. If the other country conquered you they took you as slaves if they didn't kill you. If you conquered them, you did the same. It was seen as a horrible fate, to be dragged off to another land and made to row galleys or work in mines, but there were only hints in the literature that this was an unfair thing. It was just how things were. The Hebrew scriptures started putting some dents in that. There were rules about what you could do to slaves, especially if they were countrymen. This increased over time, and the Roman Catholic Church kept chopping away at what was permitted and what was not. These seem like small things now, in an era when everyone just KNOWS that slavery is the worstest moral evil ever, especially in America, where it was invented.
Something similar happened with women's rights, which advanced in some places more quickly than others. Most places in the world there is still a significant disparity between what women are allowed to do and what men are. But it was step-by step, and occurred first in NW Europe inside the Hajnal Line. Slowly, women could inherit property and own it; women could belong to guilds; women had to consent to their marriages. It is not just a matter of "what we take for granted now," but a complete failure to understand that what we consider morally obvious was morally unknown throughout most of history.
I heard a podcast reference to someone in a situation a little over a century ago "just following orders" to excuse their moral failing. The line was delivered contemptuously, that "we've come to regard that as a ridiculous these days." Well, maybe it is. It certainly feels that way to us now. But it didn't seem so until very recently. In hierarchical societies, where group survival hung on everyone sticking together and following the leader, "I was only obeying orders" would have seemed sensible for thousands of years. If you didn't obey orders you got killed or the whole group might lose the battle and get sold into slavery. Only much later in history - again, inside the Hajnal line primarily - did the idea of being fully responsible for independent moral action start to become the norm.
It's my usual rule: always be most suspicious of "what everyone knows."