Truth is relative to time and place. It changes. Yes, we can project our own views backwards, but I prefer to try to understand the people and the place by what they considered the truth. I am a relativist in that regard. What is true now was not considered always true and may yet change again in the future in another time and place. You and I both consider slavery to be an evil. My point is that it was not always considered an evil by all the people at the time/location. It's only relatively recently in the past 3 centuries or so that slavery has become considered an evil.
We can debate on and on about this. I don't know that we will reach agreement.