Because the writer said that villians can come from abuse. And you saw the flashback that Ah-in was abused as a child. I think the writer might turn him into some sadist
I don't think he'll become a killer but I feel like he'll have a close bond with the seme because of his abuse. Like how most people who are abuse tend to be in abusive relationships.
Why do I feel like....Ah-in is going to end up being a killer too? Like I'm really interested to see what route this story goes.