Because he doesn't have sympathy at all. He's a psychopath ╮(￣▽￣)╭
I actually think he has a form of autism, he doesn't connect easily with others and only cares about those he's bonded with. I can understand to an extent of what he feels because I'm also on the spectrum, some are incredibly smart (def not me) and if I mimick other in social situations, I am sure he has too.
Psychopaths have no empathy. Jung is cold, manipulative, and distrustful, but he definitely possesses empathy. If he didn't, he wouldn't have fallen in love with Seol.
Ummm Inho is a grown ass man who cant get his own life together no one is standing in the way of him being a pianist except himself...if you'll remember correctly Jung was just living his life at school, Inho is the one that found him and decide to mess with him.(and if your specifically speaking about the hand incident as the reason why Jung is in the way of Inho becoming a pianist then that's spoiler territory but just be prepared for the whole story not just what Inho thinks happened...)
Love Inho, he really deserves some happiness in the end, not with Sul of course, it's obvious she'll end up with Jung. I hope he moves on and becomes a pianist despite Jung. One thing I can't handle is Jung being completely okay with the pain he's caused to others like Inho- even if they deserve it, at least Sul feels bad for the people she's exposed (and they REALLY deserved it!) And that's why Jung and her will never be the same. I'm somewhat sorry for Jung and his weird mental illness, hope Sul does him good, but I'll never feel fond of him. I don't like people with little sympathy.