If we're truly going to take the (correct) position that enslaving human beings is one of the most deeply and irredeemably immoral acts regardless of context, we need to reckon with the reality that the cultures we consider foundational to "Western civilization" were deeply and structurally evil and represented a tremendous ethical regression from what came before. Slavery was a critical part of ancient Greece and Rome, and inseparable from the aspects of those cultures we lionize.
That's not to say there's nothing of value to learn from them or that all Western everything should be discarded as "corrupt", but rather that we must carefully work to disentangle the good from the bad and we must vehemently fight back against the narrative that those cultures represented clear "progress" from a "barbarian" past.
