theories

i’m guessing that minjae bullied (or otherwise somehow pushed) his son to his breaking point and as a result he died. This would connect why the dad hates minjae so much, wants revenge, and claims he is living with his sons murderer. He adopted minjae with a plan in mind, so the dad had to know him before. Also, we can assume that minjae and his son would be the same age, possibly going to the same school, therefore his son and minjae could be romatically involved since he said his first love was in 6th grade (in addition could explain attraction to his dad). Now, we also know that his death was school related since the dad stated he caused the death because he pushed him to go to school that dad. With that being established, the issue of what happened still lies. Perhaps, Minjae is unstable (clearly) and kills him (which i would say is unlike if he was his “first love” and the motive of killing to be with his dad is out of the way since his first time meeting him was at the orphanage after the sons death- refer back to the dads plan of revenge) or he was bullied by someone else to death and minjae was there (maybe trying to help) and it was just wrong place, wrong time. To me though, minjae loving his dad is just a coping mechanism of the empty place in his heart he is trying to fill with someone like his son (which would be no one better than the person you share genes with). So in all, the dad wants revenge to avenge- put his sons death at ease, wants to get even, wants to make minjae suffer like minjae made his son. That correlation, however, is tricky; it couldve been one sided and since it couldn’t be returned minjae killed his son. In short, this people are insane.