He bit hers in return, a little harder than she had. Demons' tolerance for pain tended to be higher than humans', and so did their fondness for it - he didn't feel he needed to be particularly careful with this one, and frankly, he didn't want her to feel she needed to be careful either. Admittedly, she did, in general, need to be careful around him, given his tendency to smite when displeased, but this was one area where he was a lot more laidback. Often literally.