Something she just said terrified him, out of all the objectively frightening things she's said throughout this conversation. She doesn't know what it is and she can't find out from him because she can't talk to him because he expects her to be someone she's not so thoroughly that he can't tell who she is.

She lets go of the back of the chair, revealing four little crescent-shaped dents, which she supposes is somewhat melodramatic but it is shortly going to stop mattering. And she walks out of the bar without looking back.