
It explains it quite good actually
Like he was traumatiser by his dad and even his mom doesnt want to acknowledge his existence and he thought winning is the only way to prove that he is not a failure. As said at this chapter, he doesn't care about who he have sex with because its just a mean to relieve stress, but he still pay for Dan's debt and help him when he was beaten which shows he still have an ounce of care but is too naive to acknowledge it combined with his hatred of Dan being so weak and helpless reminding him of the past he hates..
That checks out

He didn't mind if they stopped. He would just change partners.
Also Dan added new things to his carefully crafted life.
Take care of the granny. Eat Dan made meals.
Take care of Dan. Feeling possessive over someone.
And Jaekyung is a bullheaded jock. Of course he'd want to fight the whole change thing off.
is it just me or does his past still not even explain how he treated Dan? and let me make clar that i dont mean "justify" (which would not be applicable under any circumstance) but i dont see how it explains it either? bcs it doesn't seem like he treated his past sex partners as roughly as he did Dan.. so wth happened?