Eh, an author doesn't need to have sick or twisted thought to think up this types of stories. They fiction at the end of the day. Author most likely just really wanted to write something ff'ed up (with characters that got this dynamic) and to do that u gotta think about ff'ed up stuff.
Often wise, the skills to separate reality from fiction is what is of importance. So like, just cuz author writes this stuff for example, does not mean they condone that stuff irl. That's what it means to have this line when it comes to fiction.
Fictional stuff stay in fiction.

this manhwa makes my skin crawl, i cannot comprehend the brain of the author and what sick and twisted thoughts can lead to a story like this, the entire subject matter is handled so terribly that it pisses me off but oh well, i do wanna see it through to the end