guilt? or that he couldn’t bear to see won being madly in love with jinhan
What I understand is that their parents put a lot of pressure on him to be the "perfect older brother" and put a lot of emphasis on him helping Jinha. Once he met Won, he started fixating on Won which then led to his obsession with only being with Won. Which explains why he beat the tutor up when Won came to him asking to escape. Jiseo also cut ties with his friends and didn't even open the present his friend gave him. At first I thought it was Won trying to isolate Jiseo but I think it was Jiseo wanting to be the center of Won's world and only devoting his time to Won. Once Jiseo found out that Won was using him only to get to his brother (He gifted him clothes that Jinha would wear, asked him to wear his hair a certain way and would only genuinely smile when Jinha was involved) It all built up since Jiseo saw Jinha as lesser than him and knowing that Won would only love Jinha it made Jiseo react the way he did. The birthday present to Jinha was Jiseo's way of confirming that Jinha could develop feelings for Won and Jiseo couldn't stand losing the man he loved to his younger idiot brother (his words). It all came to a head once he pushed Jinha down the stairs and severely injured him. After that he stopped talking to Jinha, he could have been feeling extremely guilty for how extreme his reaction was and wanted to distance himself. For the journal, I think that it gives context into his mental state leading up to his suicide. I don't remember if we got to see his note but yeah I think it was guilt for what he had become.
TLDR: Probably cause of the guilt
I don’t get it why jiseo committed suicide if in the first place he had a firsthand in their relationship with won. Aside from that I felt pity to jiseo fr