It was just some pathetic junkie... I mean, the guy who wrote this off as some lame coming of age story is missing out. Or maybe he just was looking for some horror stuff in his story.
I believe that junkie deserves every lick he gets. He was cruel and unhumane to both Shougo and Shin, an innocent. But for some reason seeing Yuu go crazy and nearly beat a man to death gives me a not in my stomach.