what is up with ayeon hyuuung??

Just wait for the backstory to develop, i doubt you'll be able to question Yuri's attitude anymore unless u heartless lol

Nope, I still question his attitude. But I guessed pretty much from the beginning what happened so maybe that statement doesn't apply to me, for that reason, then? And I ALSO think it would make someone pretty heartless NOT to still question his attitude. HE'S the one who chased after Jaerim, after all. JAERIM WHO HAD NOTHING TO DO WITH IT. And he's the one who STILL refused to tell Jaerim anything of his past, then gets mad when Jaerim does something completely natural as a result and compounds the matter further by calling him an 'ugly loser' for actually doing the very thing that he then accuses him of NOT doing? And, on top of THAT, we're supposed to be heartless if we don't forgive the same guy that refuses to come forward and sincerely apologize for hurting someone so deeply that they CRIED over it, after all that? Like I said, no, I think the ones who are heartless are the ones who WOULD forgive him for that just because of his past. HMMM. The other guy had a pretty painful life, too, by the way, plus the way Yuri acts in the flashbacks CAN be misconstrued as someone merely taking pity on another, especially since it becomes clear in the first couple of instances of bullying that Yuri isn't protecting Donghyun because he recognizes that that IS what is going on considering that he is so completely OBLIVIOUS to that fact.

as you said yuri was the one who chased after Jaerim so if he act like that right now then he has a reason mental illness is not something would go away in 3 days or so you might be gussesing what happened to him but then also guess what kind of damge it would live on a kid for god's sake yuri is still a kid that has been living all alone for years he has no one that he could turst and his past is not something he could talk so easily have a bit sympathy

Yeah, I think you're just not empathizing properly. Yuri lost his family in a completely unexpected and tragic way, and because it was caused by someone he probably never thought capable of such a thing, on top of feeling lonely and depressed, he must have also felt extremely betrayed.
PLUS, that kid seems to have moved away after the tragedy, so Yuri has probably lived most of his life with a feeling of incompletion. Have you never had something stuck in your head of what you have wanted to say or do to a person, but you never got the chance so it just eats and eats away at you??? Well. I don't know about you, but if that isn't enough to have a heavy mental toll on a...what is he, 16 years old??? I really don't know what else to tell you, man.
I know what you mean, that being shitty is still shitty regardless of anything, but Yuri is just doing what he can. He's been literally bottling his feelings for YEARS, so obviously just opening up about his problems isn't going to be an easy thing. But he's conscious of his faults and knows that if he keeps Jaerim close to him with how emotional he's feeling, he could probably end up saying more things he'll regret.
Yuri is just a bundle of unsteady rage and regret, he deserves more pity than hate rn, tbh. Anything bad he has said to Jaerim was just him bursting because he has ran away so much from his feelings he doesn't know how to handle them anymore.