this is how i understand it althought it's too hard to understand it what do u think guys ?

I think that sangwoo is a prisonner between his past and his present with bum he is kinda thinking that the past didn't go away yet and he is somehow still living in it with an indirectly way he's just like a child with a mind wish doesn't know the difference between the right and the wrong and because of his bad memories with his dad he does the wrong cuz of his father's bad influence so i can tell that sangwoo became already the dad he's scared to be without knowing that or having consicious about i can tell that his sick hobby of killing people is just like doing what his dad was used to do to him and his mother ( nothing than the pain ) but also every child had that desire of trusting people and having feelings of tendress towards them and with bum it might really became love i mean his sick mind put bum kinda in the place of his mom that's why when he thought he was going to loose him his human side was awaken to do the right thing wish it was saving bum cuz hearting bum made him remember how his dad used to abuse his mom it's just like he had a sensé of consicious a bit