QUESTION!!!

I don't think the details of where either corpse was have been made clear. The police officers conversation amounted to " Both parents were murdered when Oh Sangwoo was in school, 3 years ago. It was classed as a Double Homicide case. No evidence was found". Then we have Sangwoo's delusion of burying his dad, after his mum bought him apples and saying that at this point his mum's mental health was quickly deteriorating.

Nothing clear has been said about any of the murders, or whether they were murder at all. Be it who was the criminal, or what happened to the bodies. Regarding the mountain comment, he could have scattered his dad's ashes there, it doesn't have to mean the corpse was hidden on the mountain.

*murdered at all (as in suicide or if the death was accidental)

I don't understand why he killed his mother if he loved her

No evidence yet that he did. But if he was the person behind her death, it could have been because she asked him to kill her, or that Sangwoo thought it was a merciful thing (in his perspective) to do, after seeing her so unwell, and potentially having witnessed his mum kill her dad. Maybe the mum tried to stab the dad, got injured, so Sangwoo felt he had to end her life rather than take her to a hospital and see her end up in prison.

At this stage, who killed who no longer seems as crucial to why Sangwoo is struggling so much as an adult, but his overall relationship with his mum. This relationship, will likely provide the answer concerning his parents deaths, be they from murder or suicide.

* At the end of the day, it wasn't a healthy parent and child relationship (from Sangwoo's interpretation) but a codependent one. Where his mother was abusive, whilst also herself seemingly being a victim of physical abuse from her partner.

Maybe there are many things unclear i hope we can see th details soon, thank you!