Did you ever see the TV series Humans? It’s about robots that are made to serve humans, who look just like humans, and they can be bought and sold. The problem is some of them become consciously aware, and begin to feel abused and want out. Some of them had been used and abused, and some had been treated fairly well.
I think creating a human to be a slave is immoral in the first place. If slave people did exist, it’s still immoral to treat them as a thing, or to treat them as less than human. I think when it crosses the line a second time is if the “masters” can literally own other people or buy and sell them.
It reminds me of cultures where a woman is married off at a young age. As a wife she does all the work at the house, has to have sex with her husband on demand, and doesn’t have the financial ability to leave. Even if she is born into that custom, socialized into that custom, and even if she thinks she wants that life, I still say that is not acceptable in society.
As I write this, it’s simply wanting a slave that I find reprehensible. I just don’t understand it. I’d love to have help at my house, like a maid, or help with a business I’m running, but free help where the person has no choice but to obey me, and live with what I provide, it’s not attractive to me at all. It just feels wrong. I have a hard time watching horses have to obey their owners to take them riding, or pull a wagon. I’m not so sure the horses really want to do it.