RALPH:  Gertrude, understandably, is horrified by what's happened.  She asks Hamlet a rhetorical question: what have you done?

SARAH:  But for Hamlet, it's not a rhetorical question at all — he doesn't know what he's done, he doesn't know who he's killed, and therefore he can't grasp the significance of his action. 

RALPH:  Gertrude scolds Hamlet for being so thoughtless and so hasty — but, even as stunned as he has to be from killing a man, he comes up with a snappy response that brings them — and us — back to Gertrude's own shocking actions: did she have something to do with the killing of the King?

Sarah: Gertrude's reply — simply repeating the part of Hamlet's line about killing the King — seems to suggest that she's truly surprised by his accusation, and that she's therefore innocent of any role in King Hamlet's murder.

RALPH:  It's not clear whether Hamlet is testing Gertrude here, to see whether or not she was complicit in his father's murder — the Ghost certainly didn't say anything about her being involved when he revealed the murder to Hamlet.

SARAH:  It's almost as if, in Hamlet's mind, these two things — murdering his father and remarrying Claudius — are somehow both part of one single hideous crime, and Hamlet can't separate them in his thinking.