Question???

Yeah because it's sad what happened to him he was a victim in his life but that doesn’t justify his abuse towards dan cause at this point he is just becoming like the ones who made his life shitty aka his shitty dad

Parrots parroting, my friend, don't let them annoy you.
A lot of people are mad because the author is simply ADDING DEPTH to the fucking coprotagonist of the story. I don't know where the guys've read tons of "justification" or even "explanation" in the latter chapters; I can only see Mingwa trying to use the backstory to humanize JK a little bit more in this final half of the manhwa, which is necessary given all the wrongdoings, rudeness and violence of his.
I'm not saying it's good writing, but let's be real here, we're not reading The Brothers Karamazov as well. There are many criticisms I would like to point out at Jinx, especially the oversexualization of violent and abusive scenes, however Mingwa didn't create JK to be discarded out at the end, and this is pretty clear throughout the chapters. He's the very first character to be shown, the story travels around him and he's settled to be someone of help for Dan in critical moments.
If you don't agree with JK and Dan being together, it's ok — actually irl I would recommend any friend of mine to run away from someone like him. But that's not the story Mingwa wanted to write, so it's inevitable we'll have some forgiveness for him in the end, we can only pray for that to happen after a long struggle and a dose of humiliation for him, because — as it may seem obvious — there isn't any justification for anything he's done to our beloved Dan.