I dont get it

There was no romantical relationship...EVER...Sangwoo is just messed up and enjoys hurting others. Dunno where everyone else got the"romance" signal between them, when throughout this manhwa all the signs hint danger, brutality and stockholm syndrome. Bum liked Sangwoo at first but dunno if you can call it like when he was just a stalker whwho didn't know anything about himexcept his looksname. Bum is also messed up in his little world due to severe trauma. Liking and loving are totally different things.

Amen!

I think that Sangwoo is a totally psychopath. They often manipulate the person around them (like everyone thinks that sangwoo is good) and I think that sangwoo used bum because of his affection for sangwoo and made him feel like he liked him or cared so that bum would have hope that they have a chance together as lovers. And so bum would stay with sangwoo even if he does bad thinks with or against bum. I think this could be because he was all alone and sad when his mother died that he wants someone to care about him. eventhoug it is someone he does not even care for

I agree, with the anon above. There is no romance at all in their relationship, if there is, it's at most one-sided. I'm not an expert with Bum's BPD, but from what I understand, Bum does genuinely feel like he loves Sangwoo. Sangwoo's opinions about him matter, and it matters very, very much. He seeks constant validation of some kind from Sangwoo, he loves it when he receives affection from Sangwoo and he cares very much of what Sangwoo thinks of him. I read here somewhere in the comments that if ever Bum does run away from Sangwoo, he would come back. Bum is all about Sangwoo. That's why Sangwoo's words had so much impact to Bum in CH20 and CH26. Sangwoo walking away made Bum feel pathetic and very unwanted by his "favorite person". It's just that now he would prefer death rather than let Sangwoo think he is disgusting. (But I think his attempt to kill himself is also triggered by the flashbacks)

Regarding the killing of Jieun, Sangwoo had been disappointed with Bum. I think Sangwoo resonates with Bum in a way that he knows how being a victim feels like, he was a victim once too and he hates it. In a way he relates to Bum, so he went out of his way to find someone for Bum to kill. (Sangwoo logic lol) I think he did that so that Bum would have a taste of superiority and find comfort from it. For Sangwoo, the ultimate feeling of superiority is having a control over life and death. Jieun was a gift to Bum from Sangwoo. He expected Bum to be thankful, so imagine his dismay when Bum "feels nothing" about it. Sangwoo felt disrespected, disappointed and pissed. I even see a bit of progression in their relationship, in the earlier chapters, he'd beat up Bum for one slight mistake. Now, he resolves to passive-aggressive shit as consequence.

This is just an interpretation of mine. Others might see it differently.

If you interpret Sangwoo' interactions with Bum as romantic chemistry, then yes his behavior has changed a bit. But if you look at their relationship another way it hasn't change a whole lot. I don’t Sangwoo ever truly cared about how Bum felt. Sangwoo doesn’t keep Bum around for companionship, Bum is merely an object to Sangwoo. Sometimes Sangwoo uses Bum as a security blanket or some sort of substitute for his mother, other times he treats Bum as his play thing, but he never actually cares about Bum as a person.

When killing Jieun I think it was just a game for Sangwoo. Instead of him doing the killing that time he wanted Bum to entertain him by doing it. However he didn’t want only want Bum to kill her, he also wanted a certain reaction out of Bum from it (maybe thrill?). It was so unsatisfying to Sangwoo because he put so much effort in to preparing Jieun and Bum felt “nothing at all”, for Sangwoo that was a very boring outcome.

sangwoo is pms af.

This is fucked up