You don't think Sangwoo is in love with him in his own twisted way? He's yet to kill him despite all his other captives dying within like a day, He basically starts crying when he thinks Bum ran away, he actually fuckin' BLUSHES when Bum rubs their semen together and says he wishes they could be together like that, he SPARES Bum after the whole "I'm leaving!" scene (can you state a single person he's had patience with?). Even in this latesty chapter i'm pretty sure he is literally proposing to Bum in his own creepy way, like you say it'll keep Bum chained to him and he calls it their anniversary. He's definitely in love in my book, but an obsessive love as he is sick.
[spoilers]
okay so ive read up to chapter 19, and like damnnnn. this story is so captivating and chilling, i havent read something so interesting in a while.
its the opposite of Stockholm syndrome since Bum already loved sangwoo, and he now finds out he is this crazy guy and we see this battle he is having with himself,like he can stay and risk being hurt and killed but he'll be with this guy that he had loved for so long or he can go and be free but to what life? he has no friends and was being abused by his uncle. it's amazing to see his both want and will to survive and escape and at the same time how he might descend into madness with sangwoo.
i think Sangwoo is becoming obsessed with yoonbum, its not love but he does love to toy around with him, and now he has made him his partner in crime, he doesn't want to let go of bum and now he has manipulated him to kill, this in a way has chained bum to him and that house forever.
the whole story has this eerie and chilling feel to it. and that scene when bum opens the door after deciding to escape and Sangwoo is siting there, waiting for him, listening to that song... god it felt like out of a movie, the reader wants him to escape they are cheering him on to just go and after he finally opens the door your heart feels with dread when you realise it was a trap (although i had suspected he was sitting out there since he hadn't taken the umbrella, still an exhilarating scene)
it's an amazing physiological-thriller story, and i cant wait for the next chapter