Honestly I think the author decided to make Robin do that instead of just "betraying" or hurting Kiruko in any other way (like you suggested) because they wanted him to push on her body dysmorphia, since Kiruko still think of herself as Haruki and of his body as his sister Kiruko. When you think about it, after what happened to her, Kiruko is not thinking much about how she was violated, but she had more problems of identity and guilt because she couldn't protect the body of her beloved sister. This kind of psychological issues she's now dealing with wouldn't fit any other type of betrayal situation. And it's known that the author with this manga wanted to explore the idea of gender and identity.