
I can be totally wrong but doesn't it seem that Sangwoo sees Bum as a sort of motherfigure?

He definitely sees him as his mom.
http://www.mangago.zone/read-manga/killing_stalking/mf/c006/33/

omg this is so good but I don't understand a thing TT

So it's like there was a Sensei who killed some boys and wrote books about them. When he was reincarnated, he was meant to be reincarnated so that the boys he killed could take revenge on him. The main character was also one of the boys he killed.
They seemed to be in love with him before they started to hate him because of the killing. I don't know why, but it seems like Sensei tends to kill his partner when they say they love him...
It was probably the intention of the Sensei to write his books again after reincarnation, so that the reincarnations of the boys would find him, so that they would kill him.
I'm so happy that I've read this. The story is amazing.