First, they echo the predators who chased our ancestors. You fear the fangs of a vampire the way you fear the fangs of a wolf. That’s an external fear, a fear of the Beast.
Second, they evoke human aggression and human perversion. A vampire looks like a man and yet it is an eater of men — here’s a hunt of cannibalism, our oldest taboo. That’s an internal fear, a fear of the Best Within.
Third, we fear transformation into the Best — a chance bite form a wolf can turn you into a bloodthirsty monster, Dr. Frankenstein can reanimate your corpse with a bolt of lightening, the right combination of chemicals can turn good Dr. Jekyll into evil Mr. Hyde. We fear the monster’s capacity for evil because we can recognize it in human hearts.
From Idlewild, by Nick Sagan.