I mostly get the impression that Miss Bingley is just thoroughly convinced of her own ability to flatter and charm a man into adoring her. Her methods have probably worked often enough before to give her confidence in them. And when Darcy starts to feel an inclination towards Elizabeth, she sees this as a direct insult, ie. "how can he possibly think SHE'S better than me!"

As for her feelings, I doubt they go much deeper than infatuation. On the surface, Darcy represents "the whole package" as far as good marriages are concerned: young, good-looking, wealthy, good family, and respectable. I think she's more in love with that image--and the idea of being mistress of Pemberley--than she is with Darcy himself. In order to truly love someone, one must have an understanding of their character, and Miss Bingley's actions show that she doesn't possess that.