[johnny]

Beauty makes people drop their guard. It's sort of been drilled into our heads all our lives - through fairytales and folk tales, especially - that evil people are ugly, and beautiful people are good. Just look at Cinderella and the Ugly Stepsisters, the prevalence of "old crones" who turn out to be evil witches. And the fact that, usually, the first thing described of the protagonist in those old stories is that they're "very pretty" or "beautiful."

Vampires being beautiful makes perfect sense to me.