"You don't know that. He doesn't event know that. You're giving vampires far more credit than they deserve." A vampire's thirst for blood is something incredible, almost euphoric when sated. Losing control can happen with incredible ease, even for the most practiced.

"Do you know how many people I have killed? Do you know how many people he has killed? Have you asked him? Why don't you just date a nice human boy who you can have lazy mornings and have picnics in the park with?" It's half joking but there's some truth in it too. Things would be far easier if Nancy simply didn't date the supernatural. At least her personal life would be easier, if not her professional.