As historians, we are told (or strongly encouraged) not to let the ideas and values of our present world influence our moral judgements about the past. No where is this truer than slavery. My question is that when the topic of slavery does come up in discussions with, say, laypeople, how do we as historians talk about the subject without coming off as a slavery apologist? Obviously I agree that slavery was a horrendous institution but how do we communicate that?
The premise of this question may be flawed. Slavery is not universally reviled today, it still exists all over the world, openly. Nor was it universally accepted in the past, there are records of slave revolts from Roman times and, in the US, abolition was a significant movement well prior to the civil war.
Your question might be more anthropological than historical in nature. You seem to want to figure it how different societies view and interpret slavery without letting your values coloring that. Whether you can or should put aside your own values while learning is another question entirely.