This scenario must be a straight-up trigger for Dean, back to the forty years he spent in Hell.
No demon has touched him quite like this since Alastair, and Alastair broke him.
He looks at her as if he wants to kill her. He looks at her with the eyes of a killer.
She is threatening to violate him and force him to torture. She does not know how far down that road he’s already been; that before Cas raised him from perdition, he tortured souls for pleasure.
He’s in league with a dubious angel and a demon wants to ride him back to the heart of his own darkness.
Dean’s soul is on the chessboard this season.