What a nasty cliffhanger. I understand they were immature when stuff happened in middle sc...

Bro was force to eat a cockroach and was bullied as bad as yeonwoon. He was venting cuz he was sad and frustrated that yeonwoon didn't stood up for him like he did

Both were victims, and he made an effort to help. In a situation where one wanted to seek help and the other wanted to hide it for the sake of his parents, and as the bullying trash said, the police don't pay attention to those cases.

Or rather yeonwoo couldn't. It was evident that the situation was out of his control. Instead of blaming the perpetrator, he outright said those hurtful things?
I understand where he was coming from, it was NOT yeonwoo's fault, was it?

That's right. None of them could cope with the situation nor find an effective solution. What he did was wrong however he can't be blame for it. Under extreme phychological pressure and pain he blamed yeonwoon because he felt betrayed. However none of them are to be blamed. They were and still are kids. Good kids.

Of course, I'm not blaming him for what he did BACK THEN. But despite all that happened, he had the nerve to approach yeonwoo after l those years. What do these two have between them now? Trauma! Ofc that guy will apologize to yeonwoo now, and seeing how kind yeonwoo is, eventually he will forgive him and resolve the issue. But it's not to be resolved when their friendship breakup left yeonwoo is a more vulnerable state, not because of the incident itself, rather how it happened

I don't see the harm in clearing things up between them, even though so much time has passed and the situation has been horrible.

Perhaps it's a personal perspective. I've been through many difficult situations in my life and have learned to forgive and apologize , even years afterward. Since I live in a slum/favella and I am a school teacher, I've seen many situations like this, and I've learned that it's better to fix things no matter how many years have passed.
An example of phicologist is what happened after WWII. Some soldiers apologized to their victims, and for many, forgiveness was impossible; for others, forgiveness was a consolation.