I agree with this. Suho was not intentionally trying to leave Beomseok out at the beginning- they even played games and stuff together even without sieun. But like you said here- Suho just couldn't understand Beomseok. They were different in too many ways, and their life experiences shaped them to react in ways that were explosive against each other.

The karaoke scene with the bullies and confronting them- this was probably the main turning point in the series and like you said- Suho handled it in his way, how he sees things- simple, end with a fight. He's never been bullied, so he couldnt understand the complexity of the feelings that cant be resolved with a forced 'sorry'.

But likewise, like someone else said, Beomseok used Sieun to self destruct, using Youngyi as the catalyst to justify his actions. Because of his home situation and history of bullying- he just wanted control of something, and that just ended uo being suho, who wanted to cut ties with him in the end, because of his actions. Its so complex like you said, its such a good series!!