The thing is, the past of JK is kinda a trauma for him, not just some situation about poverty and bad parenthood. He even has nightmares about his past, so it must be really hard for him to even try changing his personality. The thing here is that Kim Dan has his own problems, and with the grandma situation everything is even worse. Jk is all about talk first and think later. He has to learn to control his temper and stop messing with Dan. He is not guilty of anything that happens to him.

I get that jaekyung is/was a terrible person but give him a little faith he’s trying to better himself and yes he shouldn’t have taken his anger out on Kim Dan but chill on him yall are doing a bit much allbeit he is bettering himself cause of guilt and cause he knows what he did was wrong but at least he’s trying some author wouldn’t care for any redemption and would’ve left him the same way bare minimum yes but something isn’t nothing