] What, really, has Ophelia done that leaves Hamlet with so deep a sense of betrayal?

Hamlet is surrounded by falsehood and betrayal. He thought he could count on Ophelia at least - after all they love each other. But, she has obeyed her father and "I did repel his letters and denied His access to me." That is a clear rejection. And then she lies to him about where her father is, a clear betrayal. And he just snaps and takes out all his hurt and anger on her. Not well done of him, but her actions (or inactions) were not well done either.