] I couldn't really otherwise account for why Hamlet was so cruel to her in his "get thee to a nunnery" speech. Does anyone know or have a theory?

I think two things happen in the nunnery speech. First is that Hamlet feels completely betrayed by Ophelia telling him her father is not there and not listening. I think Hamlet knows that he is there, and listening to every word (along with Claudius). He thinks that Ophelia has sided with Claudius against him because she feels greater loyalty to her father than to him.

Second, I think Hamlet loses his composure/temper/perspective when he thinks about all these people betraying him, and he fights against the betrayal in the way he's always fought: with words. When he is cruel, it's because he feels he has to be in order to protect himself. He's like a wounded bear fighting for its life.