
Or maybe the father killed the mother. When Sangwoo was trying to drown Bum in the bathtub, flashbacks to "him" trying to drown his mother. But it stood out to me that, actually, that man doesnt look like Sangwoo, and it resembles to the vague image we get from his dad. It kinda looks like his father (maybe im overthinking things tho).
If I remember well, they didnt found his mothers corpse, so its really weird... but we do share the same theory here.

Yeah, I was thinking something like that had happened, but I feel more like it was his mother who sexually abused him? And that's why he killed her? Or maybe he killed his mother by mistake when trying to kill his dad? Or his father killed her mother because of yout theory?? Idk I feel so damn confused (don't mind me)
I'm sure he slept with his mother.
The chapter when Sangwoo and Yoonbum left to bury the girl.
On the road, they ate apples. (The Forbidden fruit)
Yoonbum ate and right after we learned that Yoonbum had slept with his uncle (even if it's a rape). A "relationship" forbidden and taboo
Sangwoo also ate apples.
And in an interview, Koogie said that the meaning of the apple was ... a loot.
Throughout history, when the mother appears, it's always ambiguous.
And in this chapter it's just obvious, he was alone, with an abusive father, his mother was his only landmark.
So ... I think he slept with his mother, the father discovered that, and that's why Sangwoo killed his parents