I did miss something about Heesoo. Still, that does not give Heesoo the privilege to condemn Hyesung. I get that Hyesung is very much a like child, most of the time selfish but attacking Hyesung (not in the physical way) won't solve anything. Both Hyesung and Dojin need to talk. A serious proper conversation that two adults should have to discuss their problem, not by running away and left people hanging.
He does not like Hye Sung because he thinks that he is not good enough for Dojin. He idealizes Dojin, for him Dojin has to be with someone in his social league. Hye sung is poor, uneducated (for hee soo point of view), without a family and recessive so Hee soo does not like the fact that Dojin likes him. And the way Hye sung was mean to Dojin did not help.

I'm rereading from the beginning and I realize that Hee Soo was an asshole from the start. When he made research on Hye Sung he said that he was "garbage" because he dropped school and have many jobs. Who said that about someone.
When Hye Sung is about to get in a prostitution scam he did not want to help. There is people I hate but I would never let them get into a horrible situation like this if I can just call the police to stop that.
He judged him and condamned him from the very beginning without even talking to him once.