I feel like he treats Kim Dan badly because he reminds him of his younger self: powerless, poor, abused, and helpless. I'm no psychologist, but I'd say Jaekyung is angry at himself and can't forgive himself (protecting his mother, fight more against his dad, searching the approbation of his dad..). Basically, he hates his old self and now it affects Kim Dan.
I feel like he treats Kim Dan badly because he reminds him of his younger self: powerless, poor, abused, and helpless. I'm no psychologist, but I'd say Jaekyung is angry at himself and can't forgive himself (protecting his mother, fight more against his dad, searching the approbation of his dad..). Basically, he hates his old self and now it affects Kim Dan.