Slaves were expensive, it's just stupid to damage your own property , in the same way that common sense dictates that you would train a dog by kicking him , mistreating a slave goes against common sense.
It just STUPID cruelty.
The autor is just lazy and want a easy antagonisable antagonist.
Now that's just outright wrong. People mistreated their slaves plenty in history.
The USA to this day is still dealing with the cultural history of racist slave owners who treated their slaves of a different race as trash, and many to this day still see the former slave ethnicities as beneath them.
You can't say people didn't treat their slaves badly, that doesn't make sense when within the last 200 years we got racists making up shit like "well their skulls are shaped a certain way so they are the inferior primitive beings compared to us" (Phrenology), and we still got those racist people today.
Like read a book, European slave owners colonies in Africa were exactly like this but even worse. Look up Leopold 2 of Belgium, guy got 10 million people killed because of this same attitude you think only happens in fictional manga.
Human beings IRL will be this exact kind of scum, and we don't even need magic in our world or actually completely different fantasy races like elves/demihumans/etc to do it.