okay the ending is actually kinda easy to understand if you stop thinking with you dicks and vaginas. he doesn't go back to yule. and yule only went to see him because he still believes he is his. or he does still love him kyung enough to love him from a distance.
but the ending with him saying hi back and thinking of what happened in the past.
this ending means that inspite of everything that has happened to him. he knows he'll be okay. he knows if he dwells on everything that happened before that he'll just end up being the easy target he was before. so when he flashed back and said hi to yule. him saying hi symbolizes that he's moved past everything.
i'm not saying he's going to forget because that's just not possible. him saying is him saying. "i know what you did but I've moved on."
Hogu Hagyeongsu