well we did get hints that he was abused and such when he was younger, and it lead him to be a sadist essentially, but it's no excuse for rape ever.
Wait when did that happen???
I can try to find the exact chapter, but we were told of how he had a "sickness" or smth when he was younger. Give me a moment to try and find it ^^
found it! Chapter 57 page 21. Here's a link
http://www.mangago.zone/read-manga/yahwacheop/uu/iur_chapter-57/pg-21/
Basically, I think it was hinted that he was actually being abused by his father, and his father lied about it, saying it was a illness which caused these changes in Seungho. He used to be "normal", but because of the abuse he became broken.
You also can see a bruise on his cheek from when he was younger in the flashback thing. but yeah, I think Yoon Suengho is mentally broken and is abusing Nakyum as a sort of vice.
I can guess that eventually, he will "redeem" himself, but personally, with all the rape, it feels very poor and I don't know if I will ever truly like a character like Suengho. either way, it's all very sad, but it's intriguing at least, which will keep me reading for now. anyways- hope this helped!!
Seungho is going jonkers, I'm starting to believe that he has a mental illness wtf