
He simply wasn't raised properly and has no sense of morals. He got where he got thanks to his own effort and endurance so he got used of doing things his way. It's not about WHY he did that, it's about HOW he'll learn to care about other people i.e Dan, which is something he's never experienced before. Well except for the gym granny which translates to his compassion towards Dan's granny. That's about it.

He is a full adult, blaming his upbringing its understandable until...maybe 18 when he was selling drugs but now? nah and we didn't need to know HOW he turned out to be this way, we needed to know WHY (and also nothing about his supposed "jinx").
He did not abused Dan because of his bad childhood? Come on now.

You don't get it. He had a total of 0.3 adults care for him throughout his whole childhood. He has no concept for compassion and knows nothing about caring for another human being. Since no one cared for him either. In his mind the way he acts makes total sense because no one taught him otherwise.
And if we wanna interpret his case paychologically, Jaekyung created an "asshole" persona as a defense mechanism to protect himself from more shocks and letdowns. That's one way people would react when they go through some shitty situations in life. He might even blame himself for his father's death because he links it to his victory. So he chose to distance himself from people and be a jerk, subconsciously of course. Also mistreating people because that's how he's been treated, as if taking revenge. And that doesn't change neither before nor after 18 honey. He needs to get over his past traumas and heal in order to grow.
And FYI most people with psychological disorders are not even aware of them. They find their actions to be "normal". It's the people around them and their daily interactions that lead them to seek treatment. But since he's a pro at being violent and earns money from it, people just dismiss it as "his character" and don't look too deep into it. Also no close people to call him off, so how would he get any better?
As for his jinx, exactly why are we supposed to know EVERYTHING all at the same time? The series is still ongoing.

Jaekyung is NOT mentally ill baby, he is just an asshole. Also that's so low because not all people who are mentally ill do awful things to other.
Using the excuse that no one taught him how to a decent fucking human being, does not makes sense since someone did. Remember, we know his sob story from his trainer, someone who lend him a hand.
He was not a loving parent but he was a figure that tried to straightened Jaekyung's life and he did actually so Jaekyung may not have "healed" from his past but he definitely moved on. He met someone who was the same as he was and instead of trying the be a decent human being, he chooses to torture him even more, he became just like his father, picking on the weak, he chooses to forget that he was once like Dan. All of that can't be justified with "he didn't know how to act", let's be fucking for real.

To think that a coach who trained someone for a few years (was it 3 to 5 years at most?) can replace both their mom and dad is beyond me.
He's not a father figure, he's a coach who stopped looking for him when he didn't visit for training.
How can you tell that he moved on? We've known that he has insomnia from the start. And his jinx might be another coping habit he created to not feel lonely.
I don't care if you like him or feel sorry for him or whatever. This is human psychology. His past story might seem a bit extreme and forced but you can't deny that that kind of past would shape something like what jaekyung's personality is like.
And in the end it's up to you to appreciate the story as is or leave it.

Aaand yet in today's chapter we saw them talking and Jaekyung taking his advice....so is the coach an important figure for him or not? Knowing Jaekyung he would not take advice from someone he doesn't care about.
His jinx was what it always was, another mean for him to win and feel he worth something, he need that to win because winning makes him feel better. That's it.
I love how just today all of this was answered.
Jaekyung is not human bro, he is a fictional character and is subjected to a story and a narrative, we see what the author wants and she want us to feel sorry for him, she want us to justify him and I'm allowed to call that lazy writingm
It makes sense why Jaekyung turn out to be a freaking a$$hole but so? Nothing in these 3 (that's way too many) chapters justified or tells WHY he chooses(because it was a choice) to be such a d!ck to Dan.
I suggest y'all re read but if you don't want to:
1. Uses Dan's financial situation to manipulate him
2. Exposes Dan to a stranger while having s3x because he was "jealous"
3. Had s3x with Dan after he was badly injured and it caused his rear to tear.
4. Accuse Dan of trying to sabotage his match without having any proof
and that's half of it, he has no redeeming qualities whatsoever and I'm not saying this just because, I want to see what Mingwa is going to do because it's a hard one and it takes a good writer to know how to make the audience believe their deserve to be together. If after this Dan's warms up to him then this is not worth it anymore.