I do not think Sangwoo will watch Bum become a murderer. Neither Bum will let Sangwoo watch and not take revenge on him. I just believe that Sangwoo and Bum are going to reverse their roles. And I really hope this will be the case, just to see Sangwoo as a victim. It did not hurt ! XD
lol The music that played in my mind while Bum was stabbing her was https://youtu.be/ZjVx8Sguy40?t=1m4s
Now that song fits the scene PERFECTLY!!!
I still want to know more about Sangwoo's past, like what happened to his mother? Did he kill her and if so why? From what we read in past chapters, he seem to really truly love her.
When Bum was stabbing her, I could swear I could imagine the scene accompanied by classical music like "Waltz of the Flowers" or "Air". Also it's really heartbreaking that Bum had such terrible experiences in his life, it would definitely make sense for him to became a killer and this is coming from someone who's watch her fair share of episodes of Snapped!