He can't accept that he has feelings for him (other than brotherly). He's frustrated, angry, jealous & confused so, he's taking out his anger on his brother.
beom is always bullying sooyeon because he fell in love with him. but they're half-siblings, so it's impossible to be tangled in romantic relationship because yuck, it's not alabama lol jk. anyway, that's why he's crude and treating sooyeon like garbage. because in beom's perspective, if they can't be lovers, then it's better to be someone who will stay forever in sooyeon's mind. and what's the easiest way to being memorized by someone? giving them painful memories of you ofc. because humans are naturally harder to forget people who hurt them
i still dont get it why is the older brother treating sooyeon so badly if he likes him? and i dont feel an ounce of pity for him at all, he really deserves to die but i hope not by sooyeon’s hands bcs he seems to be into dying from sooyeon. and he does not deserves that lmfao