@icantnotthink
Slavery as an institution back in the day was basically prison.
Basically if you had a town of 20 or so people, and 1 person committed a crime, you could not afford to put that person in a jail, and feed them to keep them alive until they repented their actions. So the only choice they'd have is small punishments, death or to ignore the crime. If a person was a rapist, the worst that would happen would be they'd have to go without dinner basically, because that person was a valuable worker.
That's where slavery came in. Slavers came and took these criminals, and paid the townspeople for these criminals. The townspeople would use that money in another town to buy food and stuff, while getting rid of their offending person.
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Obviously any business that has the profit motive of needing able-bodied workers will naturally get corrupted through capitalism.
I'm not trying to defend slavery here.
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Thing is, in these stories, owning a slave isn't as world-shattering an idea as you might think. It's either you own the person as a slave, and keep them alive, or free them. Set them out into society where they have no means to support themselves and wind up in slavery again. None of the isekai slave owning protagonists ever keeps a socially well-adjusted slave, or a slave that would make it on it's own in the society they live in. If it's a beastman slave, there's a lot of beastman racists out there, that if you DID free your slave, they'd never find a job and would probably wind up in a shit position in a week minimum.
Slavery is BAD. Heck the heroes often understand this, and take steps within their ability to mitigate it, but social change isn't easy.