I don't think, Sangwoo is regretting a thing. He planned the whole thing.- brainwashing and breaking Bum. Flirting with a girl in front of Bum, people looking down at Bum, having sex in front of Bum -> Bum falling right into Sangwoo's web. Now there's no where left to run. They're both killers and Sangwoo has Bum in the palm of his hand.
I don't think that either!!! Sangwoo is really lonely inside so, in the Bum, he found someone similar to him, someone that loves him anyway, and he wants to make sure that Bum is never gonna leave him -as everyone else, even if he killed them!
Sangwoo set up the whole situation to "awake" Bum and make him join the dark side once and for all. Bum was "break" inside a long time ago and Sangwoo knows that. And he knows how to play with Bum's mind!
The only sure thing is that this story is excellent and that all this, is not going to have a good end!
Sangwoo has a look like thinking: "Oh, God!!! What have I done? I created o monster!!!"
Bum is totally in a mental breakdown! I don't think that he can be saved anymore. He crossed the line, but I can understand his situation. He is a problematic man with a scary past. He didn't receive any love as a kid or as a teenager, and he became a loner with so many issues that he couldn't talk about them with anyone.
And so is Sangwoo!!! I don't try to find excuses about their actions, they are horrible, but in a way, I can totally understand why they are like this. Probably, if their lives was normal, without any violence and bulling in them, they were going to be fine. I feel sorry about them, in a creepy way. :(