If that is what you got from this, you’re far less mature than Kaoruko. Subaru obviously didn’t go behind her back maliciously, even Kaoruko and Rintaro recognized that. Subaru was just looking out for Kaoruko, maybe not in the right way but her heart was in the right place. She rightfully apologized even before Kaoruko found out. A bad person would only apologize after the other person found out and would make excuses. Subaru never blamed Tsugumi or Kaoruko, she blamed herself and called herself a bad person for doing it. A bad person does not think they’re a bad person usually.
No one is saying that going behind another person back like this is okay, but asking for forgiveness and being open about what you did that couldve hurt that person is a good thing. I could understand having your perspective if Subaru had waited for their relationship to fall apart THEN she apologized, but that didnt happen, so it doesnt make sense saying what you said.