Ch. 40 spoilers

I don't think he killed his dad, he just moved and buried the body for his mum. I''m guessing the mum took her own life. In regards to the sexual abuse theory, Sangwoo as a child being exposed to seeing his mum having sexual activity with his dad (consensual or not) would still be classed as abuse. Who know at this stage really though, I'll buy the chapter later, I prefer the plot delving into Sangwoo's character as opposed to the focus being on Bum.

Here's another completely out there theory to add fuel to the fire: We are presuming that Bum is a replacement for Sangwoo's mum, but maybe Sangwoo's own mental illness causes him to see himself as his mum. Meaning he said "I killed my dad", not because he actually murdered the guy himself, but because his disorder causes him to view himself as his mum. This might also explain why he feels scared and traumatised at the prospect of taking a more "submissive" role during sex.

.
.
.
.
.
.
There was nothing in this chapter that implies that he witnessed them having sex.
And there's still the problem that Sangwoo said something like "I haven't killed a man since the death of my dad". So he either killed his dad or another guy when his dad died. Or it's simply a plot hole.

Hmm, I don't really see it. I mean he would've called him his husband and not his father when he talked about it then. Besides
.
.
.
.
.
Bum looks like his mum. I think it's pretty much confirmed after this chapter that Bum is supposed to replace his mum.

Not sure what age he would have been, but he would have still have been a teenager when the death happened? If he was "involved" in the murder by witnessing his mum kill his dad, maybe them both communicating to him whilst this happened, and then he moved and buried the body, from his traumatised perspective you can maybe see why he as a teenager thought that he had some part in killing him. At the end of the day, both men aren't reliable narrators, hence all the out there theories and presumptions.

I just wanted to write something like this. That's the only explanation I can think of that actually makes sense to me.
And now that I think about chapter 40 again
.
.
.
.
.
It's possible that at the end of the chapter his dad was already dead and they buried the body. So maybe he really was involved in the murder or killed him himself.

I was like (๑•ㅂ•)و✧ and then ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄ and then (⊙…⊙ ) and then Σ( ￣□￣||) then Σ(っ°Д °;)っ and ヾ(❀╹◡╹)ﾉ~

His mom soo pretty, i never thought this would be soo sad.. a bad people are made from a good people who get hurt

I think he's just replicating things he saw/heard when he was with his parents, rather than taking a specific role, but it's interesting to theorize. The proposal that he gets his identity mixed up with his mum's, isn't that he literally thinks he is her all the time, but maybe in times of extreme stress he dissociated, seeing as this has already been used in the plot regarding Bum, only we see now that Sangwoo's delusions concern his mum.

I have a feeling mom either killer or hurt Dad. In the end we see her dancing with a phantom Dad, so it’s possible that she just injured him and left him in the basement. We know that Sangwoo hears (and possibly sees) ghosts of his mother. I’m guessing Mom gave Sangwoo his mental issues.

There’s also the fact that we remember Sangwoo drove up to the mountains with his mom before (something he recreates with Bum) and leaves the apple up there after Bum ‘stepped on his dad’.

i believe he was recreating what happened with his dad, only with Bum (who reminds him of his mom) and Jieun (who ended up loving the complete wrong person - possibly like him Mom?)