Uh, my understanding is that the dude basically has depression and trauama thats linked specifically to his experiences with his sexuality. Whether its his parents both abandoning him (including his father throwing him out) ,or his ex wife divorcing him once someone sent her an old picture of him with another man, he believes that everything he does will hurt someone and end in unhappiness. His depression/trauma/negative mindset made it so that he doubted his relationship with Eunho. He felt a debt to Eunho's parents for being ao kind to him and being like a substitute for his absent parents. He was already ashamed of being gay because of the social issues that accompany it, hence his avoidance of romantic relationships, o he felt terrible for dragging Eunho, the son of these very nice people, into it with him. Eunho's own sister came up to him to tell him to leave Eunho alone. For him, that really brought his pessimism into reality. He thought that the pain of breaking up sooner rather than later would be for the best. I mean, he's hardly mentally stable. We know he took sleeping pills just to get to sleep at night. He was cruel, but its not as if he's not allowed to break up with someone.
My 2nd time reading this and I still can't understand why jaeha ended their relationship the first time