I think it’s more-so that Sangwoo’s memories have been morfed into something unlike the reality of the situation. He might have made up stories in his mind to make his mother a martyr and his father a monster, regardless of the truth of the situation. Little kids tend to misremember things, and by adulthood, those false memories are the reality for them.
I think it’s a similar scenario to now. His mother was probably abused and forced to love the father, which is why he said “you said we can be a family,” however, the mother had just as crazy tendencies; kind of like the relationship between the boys now.
I don't buy this. Such a good person wouldn't just turn into that abusive scumbag because of a cheating wife. If he did see her try to kill sangwoo it makes no sense that he would start beating her and making the kid be with her, eat with her (on a smaller table???), overall distancing his son from himself. The only logical explanation is that that man shouldn't be the same as this one. But then the time doesn't add up. AND in the light of the mom being crazy we have never seen the dad be abusive, yet sangwoo already knows he is. I just don't understand