Responses

Omg I think this is it , cuz when sangwoo found that woman on the ski trip and she was asking to save her son he was like"you aren't even blood related so why do u care so much about him" and while he was killing her he was shouting about his own mom and how she was a bad mother probably because he saw how this woman was willing to sacrifice her life for her son even if they weren't blood related ,so he thought about how his mom didn't do that for him.
Anyone feel that Sangwoo isn't blood related to his "mom"?
1) She tried to kill him (and seems to have something bothering her all throughout regarding the family, since he was young). A few reasons I can come up with regarding why she'd dislike Sangwoo to that extent would be a) he's a burden, which doesn't seem so because the dad had been providing for the family and seemed to be a caring person in the period when she tried to kill Sangwoo.
2) Dad yelled, "We agreed we'd be happy together!" after saving Sangwoo. What did they agree on in regards to being "happy"? Why'd he say that in a situation pertaining to Sangwoo? It leads me to suspect that the dad may have had an affair, had Sangwoo, and in the end the mom agreed to raise Sangwoo with him. (She does seem to be the complacent type, especially if you mirror her to Bum...) Or for one reason or another, Sangwoo just isn't her blood related son.