Hm, I see where you’re coming from...
Although, I think the mother was actually alive on the time of burying the father, considering Sangwoos comment about how they went hiking up that mountain one time with the father “lying at the back”.
In line with what you’ve said, I also think that maybe the mother actually killed herself up on the mountain afterwards... and all they saw was the two bodies left up there, and that lead the police to believe it was a double homicide.
Also in terms of this as well, maybe the mother just wanted out and wanted to leave both of them as Sangwoo keeps saying things like “are you going to say I’m just like my father and leave me too?”
There are so many theories on this it’s crazy, but I have a feeling Sangwoo wanted to help his mother and still he loved his father, but less and less once he started the beatings. But yet, he was also afraid of his mother as well? Since she was changing more and more everyday and it was in a crucially bad way for Sangwoo...
I also want to know what made the dad turn so physcotic, seeing as in the beginning of the new chapter he was a nice loving father and husband ??
Or what else do you think as well?
True. It got me confused too when there's a part where sangwoo and his mom buried someone when i remember that his parents were killed together. . well whatever, this story is cynical but i'll go along with the story first than finding holes and think what the hell.
I'm almost sure that his mother's secret is not that she killed her husband (or better not yet).
I believed Sangwoo when he said he hadn't killed a man since his father's death, but it's true that Sangwoo may not be a reliable narrator: we still don't know if he killed his father himself or just buried the body. I'm not saying his mother didn't kill his father (we don't know yet) but it was not the "secret" of the last panel. I suppose his father died not long after that episode, but at the end of ch.40 he was still alive and the "secret" is the reason why they both died.
In ch.2 the police officers (Seungbae and the other one) were talking about Sangwoo's parents' death and they refer to it as to a "double homicide incident" in which, according to Seungbae, the signs of a break were even too obvious.
Now we don't ķnow exactly who killed who, but what is almost sure is that they died together on the same occasion. Probably Sangwoo was the one who simulated a break in by an hipotetical outsider (since it's something requiring a lucid mind).
The missing point is the dynamic of burying his father's corpse: was he buried before or after the police discovered the murders? If before, how did they know it was a "double" homicide? If after, why in the first place choosing a hidden place in a wood in the mountains and not a cemetery?
I hope it's just me missing some important detail and not a plot hole. If someone has a better theory please let me know