I've just noticed this "pattern" thanks to a post from someone I follow. So,I mostly read manhwas and that person does too and we both share a lot of opinions, One of which is Rashta, from the remarried empress having too much hate. Which brings me my main point.
Why is slavery so normal in manhwas? Yes,"normal". Sure,it may not be the author's intention. The author puts slavery as a theme in these stories and rather than address it, it's just a plot point to make the protagonists shine brighter.
In a lot of manhwas, when the heroine needs protection, she'll go to a slave market and purchase an able bodied slave solely for the purpose of protecting her (though she's rich and can hire a skilled bodyguard). Now,that slave, who should be traumatized or even,scarred due to the abuse they suffered will for some reason cling to the protagonist and will eventually become obsessive slaves and then, villains.
I'm not supporting their actions. Some of them do terrible things. But,why are the slaves, not slavery or the protagonist who saw a fellow human and purchased them like a commodity so often villains? Don't forget the slave masters,too who obviously abuse these people.
I know it's supposed to be historically accurate and all but slavery is a real life thing which still happens today. So bad that some people never recover from it.