Okay, first of all.
Why isn't Yoonbum friggin psycho as Sangwoo?? He's been through a lot than fackin Sangwoo!! His ma died, he's been (sexually) abused, he's never experienced love before. WHY ISN'T HE THE ONE THAT'S KILLING PEOPLE?!
Believe it not, what his uncle and grandma did was domesticate him, his classmates made him have even less self esteem to the point that he wanted to die, and Sangwoo is the only thing that kept him together for the least amount of very f-up years.
I think Sangwoo had undergone a more brutal experience than Bum's. I'm sure this is not everything about his past. Koogi had only exposed a glimpse of his childhood and his father's abusive trait. But other than that, there hasn't been much explanation why he decided to kill people.
OMFG TEAAAAARRRRSSSSSSS (╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)(╥ω╥`)
O SHEITU