How do we feel morally/theologically about the allowance of slavery in the past?

Salam aleikum, I read and watched a lot about the topic of slavery in Islam in the last days. Besides the (r)evolutionary restrictions for the masters and in the sources of slavery, and in addition the humane commandments from the prophet (saw) in taking care of slaves, slavery was still allowed to happen in conditions. Rather the prophet (saw) set impulses in direction of human rights of slaves in light of the given time (there are not a similar cases in human history until that time). Norm and moral feelings of a society can change. In the past slavery was considered normal. So the given explanation could be, that slavery is seen as an additional hierarchy in society besides other hierarchies (as freedom is always relational) and not bad by its nature, but the conditions of slavery can be bad. At the end of the matter God allowed some type of slavery and God‘s morals can only be right. He did not abolish it completely. That would support this view. But in reviving our God given Fitrah: Is there a morally explanation (also from scholars), that illuminate on the theological matter of slavery. How do someone explain the people of today, that it is (in Islam) not evil? It could be explained in distinguishing slavery or in other ways. Thanks for responses. May Allah bless you.