Yuri's anger (and eventual hatred) towards Dong is justifiable. Receiving death wishes against him and any one he cares about is god damn awful. I also want to point out though Dong experienced severe physical, mental, and psychological abuse throughout his flashbacks. Tacks in his shoes, dead bloody rat hanged in his locker, kicks and punches where people least expect, a physically abusive father, vicious slander and vocal lashings directed to him, etc.Was it justified for him to say those horrible things to Yuri? No, it was. You can at least understand this kid eventually reached a point of a mental breakdown after those traumatic experiences.
Honestly, I blame both of them for their shitty ass communication skills. A lot could-have's and should-have's if things were done differently.
Its understandable that dong gyung was angry and thought those things about Yuri but it's also more than rational for Yuri to be angry at someone he thought of as a friend saying he hopes that Yuri and his family dies!!!! There's legit no excuse for that ! Yuri was always kind to him and willing to get into trouble for him and he actually didn't know about him being bullied as he rushed to try and help him when he found out but instead of being thankful, Dong gyung wishes death on Yuri's family??? 진짜??? 아 괜찮아