Vampires do not need to kill to feed but sybille seems to kill as someone says that being executed by the headsman is not the worst death you can have. when you ask about this he says to stay out of the dungeon when Sybille has had a bad day or something to those words.
Maybe she somtimes feeds without killing and other times kills them while feeding, after all, its not very smart for her to always keep killing prisoners when she has to feed. It would raise red flags for people who do not know her nature.
Though its possible she only kills them, not really because she has to feed, but she doesn't want more people to know she is a vampire (im sure a few already do), after all, if she fed on a prisoner and kept him alive, and if he was released, he could try to alert others that she is a vampire.