I hope this doesn't sound rude, but did you mean Sungha's extreme possessiveness towards Sooin? Because I don't remember Joohyun being possessive of Sooin unless I'm a goldfish and forgot the plot already HAHAHA (I apologize if I'm wrong T.T)
Also, I think Sungha acted that way towards Sooin because of his internal homophobia? Like he didn't want to acknowledge that he liked Sooin but at the same time didn't want to allow Sooin to find someone else to love because he secrelt loved Sooin. So by threatening, blackmailing and abusing him, he doesn't need to admit that he liked Sooin, but at the same time he can "have" Sooin to himself, he knows Sooin won't leave him because he is too afraid of Sungha.
(I hope that makes sense I'm actualy trash as explaining haha)
secretly*
Well based on my understanding is he ACTUALLY loves/likes Sooin but he has this internal homophobia ... then w his history w his Dad and then his mom dying he makes himself believe everything is disgusting, he's denying reality that he WANT Sooin, used to like/love Sooin too but he makes himself believe that its ONLY Sooin whos disgusting for for liking him .. its more like he like sooin so much he hate him at the same level... who would even have thought of wanting to ruin a person both physically and mentally? Plus hes obviously a sadistic psycho
I didn't understand Joohyun's extreme possessiveness towards Sooin(or maybe i'm forgetting something. I hope someone can explain to me), maybe his family was a part of his insanity but the shit he did to Sooin just seemed so unreasonable and fucked up. I honestly think they would've become adorable highschool sweethearts if it weren't for his insanity. I'm just gonna think that it was necessary in order for Sooin to meet Chi-hwan.
Overall, this was an amaIng read, really got me hooked you know. I love manhwas like this—with the detailed plot and characters, smut and etc. Although if you're sensitive about SA and abuse maybe you should skip some of the first parts in this manhwa.