You have asked a very good question. You also have to ask if Suho had allowed Beom Seok to get revenge on his previous bullies would the ending of this drama have happened? Also, why is Si Eun allowed a weapon and not Beok Seom?
Beok Seom is the product of his father's teaching. I will literally beat you until you fight back. He is also the product of the educational mindset of if there is a problem transfer schools. He is also the product of the mindset that this is just boys having fun. Well who is having fun? Finally, he is a product of money and power. Consequences don't mean anything as they can be covered up by transferring overseas or to another fancy school.
Suho, on the other hand, is a product of a family who seems to love and care for him. He has a job and responsibilities. Everyone in the school, including the rich kids, are scared of him. He has power with no money.
The scene with the boys at the karaoke bar was to set up Beok Seom to transition over to his class status and fulfill his father wishes. If Suho had been a great friend like every viewer thinks he is to Baek Seom, he would have helped Baek Seom get revenge, and also keep BS from crossing the line. However, he didn't, so it appears that he will defend Si Eun and allow Si Eun to fight his owen battles, but not BS. This just makes BS feel like a total loser.
I hope this helps. Many viewers are angry because they feel Baek Seom betrayed Suho and Si Eun. I disagree. Suho betrayed Baek Seom first. Thus, leading to the final show down. jmo
