Responses
Rather than being oblivious, I would say that the MC was scared. He remembered having the same look in his eyes when he was being bullied, which is why he unconsciously avoided Suwon when Suwon was being bullied. Because he understood that the ML is not someone to be messed with, even though he doesn't like the ML, he can't help but go along with what the ML says, fearing that he would be bullied again. I agree with you in the sense that I also wanted the MC to defend Suwon, especially because Suwon was his first friend? However I don't think it's so easy for him to bravely defend someone after what he had been through...
He was like oh yeah I sense something not right about this dude, watch he bully a student, noticed the the same pattern from what he was experienced before, then proceed to not see and hear (I read from another site). You saw the red flags from miles away, the irony really.