Ah. I think I’ve got it. I was struggling to understand why Hamlet suddenly starts swearing by the Rood here when he was not sworn by this particular thing before. But now I think I’ve got it. It’s a pun. Rood really sounds just like Rude. If you don’t see it written (as an audience at the time definitely wouldn’t have) they are interchangeable. In a discussion about appropriate behavior – that doubleness of Rood/Rude comes in quite handy.