"A femme fatale, translating to "Deadly Woman" in french, is an alluring, seductive woman whose charms ensnare her lovers in bonds of irresistible desire. Often this leads them into compromising, dangerous and deadly situations." ~ Wikipeda
The vampire Dracula, has made Lucy a femme fatale due to the sexual orientation that comes from his extortion of her blood. Marking her as a sexy, desirable woman that he himself has condemned.