Complex sentence: The human servant should only die when the vampire they are bound to dies, although Anita has proven that wrong on several occations, in the third book Anita lives on as she "kills" the vampire she is bound to.
Split into simpler sentences: The human servant can only die when the vampire they are bound to dies. Most human servants die as the vampire they are bound to dies, but in the third book Anita lives on as she kills the vampire she is bound to.