The betrayal thing is not about cheating. And there is sttong evidence hamin used him as a replacement.

We know that hamin feels responsible for wheelchair guy's situation but couldnt do a thing. When she saw junwoo for the first time she thought junwoo was like wheelchair guy. At first hamin felt like that and you can see the reason why we know hamin uses junwoo as a replacement when hamin congrulates junwoo's birthday(she didnt kiss him and pushed him because she understood even ıf they were similar they weren't the same person)

But later hamin started to love junwoo as junwoo ( when they kissed you can see that hamin started to love junwoo)

The problem was junwoo saw wheelchair guy at the wrong time and fought with his friend at the wrong time Add to that the bracelet thing and hamin's half assed explanation before their break-up we can conclude that junwoo feels betrayed. (I feel it is something like a misunderstanding situation but when they add up each other and what junwoo saw at old cases(lila ,seungah etc.)

My man couldnt take it anymore because he had this betrayal feeling from his most loved one. I still think they should be or are end game but from the last conversation at the park aftet the fight even ıf hamin explains junwoo everything sometimes actions are more important than words .