Her expression is an open book: first, a low furrow at her brow to show she continues to disagree with him labeling himself as a monster; second, shock, and then realization as she puts what she knows of him already with what he's recently revealed.

Not only had he needed to sever his emotions, but part of that necessity had also been to kill his father. Does it have to do something with the sword he mentioned? Hellfire? Something else...? She knows she mustn't ask, and so she quells these speculations from being voiced on the spot.

Thus, she's struck silent for all of a few seconds. How ignorant she is, truly, to so much about him.