People complaining about Bum not leaving when he was given this ~amazing chance~ do realize that if he had tried to go then that would’ve likely been the end of the series, right?
Not because Bum would’ve been free, but because he would’ve been dead.
It has been repeatedly made a point that Sangwoo’s house is in a basically dead neighborhood. You can scream for help there without anyone hearing it. That’s likely why Sangwoo was so comfortable with presenting Bum the opportunity to go once again, even if he had failed the test and gone outside he wouldn’t have made it far.
It’s not like this was the first time Sangwoo has allowed Bum the opportunity to leave (the incident with ‘crossing the line’, at the store, at the bar). It’s not because he’d want for him to go, obviously, but because he keeps testing if Bum will. If he does then Sangwoo hasn’t been successful in training him and needs to punish Bum.
When Bum crossed the line, Sangwoo was obviously furious enough that he came close to killing Bum. Since then his tests have proved more fruitful as Bum hasn’t tried to leave him like that again. But, when Bum was hiding from Seungbae and Sangwoo thought he had left, his immediate reaction was that he’d need to find Bum and kill him. He explicitly reasoned with himself that there was no other way now, so it’s clear that “Bum leaves” equals “Bum dies”.
After having so much success with Bum and things going how he had basically wanted them, Bum disappointed Sangwoo by not reacting like he was supposed to with the murder. If Bum had in that moment disappointed him again then there’s no way Sangwoo wouldn’t have killed Bum on the spot.
There’s no way Sangwoo would let their relationship end with Bum just leaving. If he truly decides that he is done with Bum, then Bum dies, simple as that. He hasn’t given up hope yet that Bum will live up to what he wants Bum to be.
Does Bum realize that? Maybe, maybe not. After all he did try to leave while Sangwoo was on stage, so it’s within the realm of possibilities that he has been able to reason that he needs to try to escape when Sangwoo doesn’t expect him to. But considering his mental state in chapter 20, his refusal to leave is not likely to be from a place of reason like that.
Either way, good job on not failing the test, Bum.