] That said, I'm still left with some questions. What, really, has Ophelia done that leaves Hamlet with so deep a sense of betrayal?

Well, some contend that his treatment of her is unconscionable, full stop.

Others contend (Maximillian Schell, who played Hamlet on the German stage to great acclaim, for example) that when Ophelia says, "At home, my lord" to Hamlet's question about her father, that she sides with the King and Polonius against Hamlet, and that Hamlet knows it and that's the end of his trust of her and anyone else.