Your logic left out the part where Sangwoo has the strength to snap Yoon Bum like a small twig. Blackmailing Sangwoo is unlikely and honestly just naive thinking. I don’t see that happening in their dynamic because Yoon Bum knows about Sangwoo’s hot and cold mood swings, and he flinches in the smallest raise of his voice. Bum can’t talk to him about stuff, let alone attempt at blackmail lol.
If he was smart and brave he'd be dead long ago... he'd be just like any of the others dead girls (like the heart sticker girl, damn that girl sure had guts). The fact that he appears so un-smart and un-brave was puzzling enough to catch Sangwoos interest
You’ve seen Yoon Bum attempt to get help from people in public. Not really successful, was it? Moreover, I know a lot of readers see Bum as cute at times, but in the manhua, he’s seen as having a disgusting, gloomy appearance. To us, he’s adorable sometimes, but not to the people in Koogi’s world. Compare him to Sangwoo, who’s well put-together and built up a good reputation. No one is going to listen to the mad ramblings of a stalker who’s been arrested multiple times before. You tried, though.
Also, last time someone tried to be “smart” and “brave”, she ended up getting stabbed multiple times. It’s a nice thought, but boring and predictable. That’s what the previous victims were: boring and predictable with their blackmailing. Bum is interesting to Sangwoo because he isn’t naive enough to copy what the previous girls did. You really are naive. You wouldn’t survive in Sangwoo’s basement lol.
Lmao you are the naive one here, there are multiple ways he could escape while he was letting him alone in public.And i said that he could try and do that if he wasn't 'sick', since it's clear he has some kind of mental illness. And you know nothing about me so don't you even dare to judge me. I don't remember calling you 'stupid' just because i don't agree with your opinion while you called me 'naive' .
And okay, let's put the blackmail away since like you said no one would listen to YoonBum with his appearance, still there are multiple ways to escape in public. And Please don't be rude or judge people in the future just because you don't agree with something.You can simply say what you want to say avoiding to call someone anything. I respect your opinion but you have no right to call me 'naive'.
in my eyes bum int smart exactly but more logical because he knows that if he did tell the police he killed the girl, both of them are charged with murder and are sent to prison. Sure he's sucking up to sangwoo, but if he did not fell for the Stockholm syndrome, bun would've had the chance to leave sangwoo at his weakest state, where he believe bum is on his side, and once he does leave, he can go to the police and tell his story of being a victim and twinge some of the events to make him even more of a victim. But then again, psychologically speaking, if bum was smart, he would've left his uncle's home and the whole events of killing stalking wouldn't happen. But because bum already has mental problems before meeting sangwoo, such as little capability of social skills, already makes him an eyesore to the society and he wouldn't get much help, before and after meeting sangwoo.

Ok, what if Yoonbum was actually 'smart' and brave and said to Sangwoo that they were through and that he had enough enduring his shit and left and then Sangwoo blackmailed him saying that he'll give him out to the police for killing that girl. Then Bum saying that he was gonna do the same with ultra confidence so if Sangwoo wanted his own secret of being a killer kept save he has to keep Bum's secret and then Bum walked out of the apartment like a bad ass? Leaving Sangwoo in a crying mess? (and he gotta do that while they're in a public place so that Sangwoo won't be able to confine him)
Don't get me wrong, that was just the logical part of my brain. I'm actually a fucked up, messed up sadistic person so i don't want that to happen of course. I still want them together :)