I was in an AD&D game a couple months ago with a group of older people (45-60) and there were 2 slaves being played as NPCs, being controlled by the DM, but owned by the other Player Characters. There was a Paladin in their group. When I showed up, with my Lawful Good character who had the "Free Spirit" character trait (basically the anti slavery super power) my character was shocked that Good aligned characters, including a Paladin, would think this was kosher. Now, stepping outside of the Game, we get to the meat of it.
The Players thought it was perfectly natural to have slaves, if the slaves were under conditions they thought were good. Note that I've been rewatching Game of Thrones recently, and the character "Reek" is a slave who has been brain washed into loving the man who tortured him and mutilated his body.
Is the guy who tortures and flays people while laughing, is this man Good because his trembling victims say they love him? Isn't there some kind of Stockholm Syndrome at play? Is the terrorist or kidnapper rapist cannibal a good person because he convinces the victims their conditions are good, when the contrary is true? Should we pat the slave master on the back because he feeds his slaves two moldy pieces of bread instead of one moldy piece of bread like his neighbors?
I can see how Child labor sweat shops, illegal immigrants and wage slaves are all just extensions of this mentality. Eventually you get to Feudalism and landed Lords who have rights the peasants do not. The Samurai chops off your head because you didn't bow and scrape while he was walking down the street. How dare you.