What is it about Fanny Dashwood that is so infuriating, so evil?

The "bad guys" in Austen (usually women) are so evil because their evil is covered over with a veneer of good manners and gentility. Mrs. N. of MP may be the worst because she practices her evil on a child. But Fanny D. is a close second. Their brand of evil is so insidious that the less perceptive in the novel don't even see it. This is, perhaps, the most common kind of evil, one we find in our own lives. This is one of the things I love about Austen. She can portray a character that seems over-the-top and yet, we have to stop and say, "Well, I actually know someone who would do that - or has done that."