The only thing the past showed me was that Jihyun was a rude asshole since the first time he met Chiwoo. He skips schools and just cause problems in general since he was young. Chiwoo being nice and friendly to him didn't change anything. He just tried to drag Chiwoo down to his level of misery.
He is a bad person, but being one doesn't make you a psychopath not even being a killer makes you one.
I don't think he was neither born this way nor that he is a psychopath.
It's is expect for children and teenager to behave the way he does given this he was abused.
Child abuse is not only physical but also emotional.
I despise Jihyun, but people are made not born.
But was he beaten cause his father is terrible or was he beaten because Jihyun himself is terrible, like skipping school and getting into fights, causing problems, etc. Not saying it's right but some parents get physical when they discipline their kids. I grew up in Asia and where I came from, we get beat for acting up. Truth is, people are assuming he's that way because of his parents or family, but we simply don't know. What we do know that we never see Jihyun at an age where he was just a nice innocent little kid. The youngest age we see him is from this flashback. And in it, he was already a jerk ass, and maybe he has always been that way.
Actually a parent does have a part to play in how your child grows up. People assume that it's mainly the child but it's about your environment. Who's around you and what you learn. Just because it wasnt disclosed doesn't mean that his father probably didn't abuse him from before all of this. From this interaction/flashback we can understand that Jihyun nerve had a healthy relationship with his father and that means he wasnt gaining any attention or love as a child. Also no one else around him was also giving him the same thing. This caused him to act out by being a rude little shit but because Chiwoo actually TRIED to understand him Jihyun started to get that craving of a child wanting attention and affection, as he stated. This caused him to be obsessed with Chiwoo and not wanting anyone to get close to him. Jihyun just doesn't know how to Express his feelings correctly cause it's not like he ever got a chance to. Their relationship is super toxic because of this and Jihyun needs therapy, or anger management classes.
Jihyun was toxic even as a child lol people alway love blaming the parents for the outcome of a child when that is not always correct. And I'm not saying his father is correct about how he handled the little psychopath/narcissis either because there is always a better way to handle children. But I think Jihyun was just born that way.