"I'd call their sins even" I disagree. Sungho did way worse with kidnapping Dohyun and repeatedly raping him and breaking him mentally. Dohyun only seduced Sungho and took it on a more physical path than just best friends or platonic lovers. But he didn't rape Sungho and remember, when they first had sex, Sungho was the one who initiated it. And all the other times when they were in highschool, there was consent on both sides. Until Sungho became disgusted with himself and stopping his relationship with Dohyun.
So to conclude: I don't see any massive "sin" done by Dohyun. He just wanted to be loved by someone but didn't rape or was super forceful towards Sungho.
uuuuuuuuh nah Dohyun used emotional manipulation on Sungho and abused Sungho's attachment/feelings for him. He also definitely raped Sungho. Im not sure what you classify as rape, but in my criminal justice class rape is rape when coerced to consent. Dohyun used threats and manipulated Sungho to unwillingly consent. That's rape.
I definitely get where you're coming from with the fact that Sungho's revenge was in fact a lot more violent and in terms of damage trumps Dohyun's coercion, but just saying, as someone with BPD, I don't really like to belittle the potential damage I can do to people around me. Emotional manipulation may not be as brutal, but essentially it is cruelty. Psycological torment triggers the same neural receptors as physical pain. Add that to the fact that Sungho was somehow unstable on his own already, it's not really my place to judge whether his pain is significantly less than what he puts Sungho through. Some people are better equipped to one kind of pain than another. Regardless, when I said I'd call their sins even, its not saying it in "eye for an eye" terms, I'm saying that they've both now have done significant damage to one another.
Think Donghyun has BPD. Being painfully familiar of the symptoms myself, seems about right. The warped self sabotage of relationships and the abandonment issues, the desperate cold front put up to avoid pain and the ability to forget on command. Its really scary to see it in this context because this is literally the worst case butterfly effect here. I have no idea what mental illness Sungho has that propagated his bad experience into full blown insanity but yikes. And just my opinion here, sure Donghyun used coercion but what Sungho made him go through was brutal. I'd call their sins even but I don't really think there's a point. Sadly, the two of them have already lost their minds to a world of pain. : /