In this new life, there are two things she knows for certain: how to kill bad guys, and that something awful must have happened to turn her into a blood-sucking monster. Unfortunately, having no memory and a sizable dead count paints a mighty big target on one's back.
With a price on her head and nowhere to run, choosing between indentured servitude with the Night Watch or certain death kind of seems like a no-brainer.
If only there wasn't that silly little rule about not killing people