The Bible states that the evil one masquerades as an angel of light: beautiful bewitching Mary. (my paraphrase) and that Miss Austen exposes much more of what motivates Henry so he is not quite such a mystery. Mary, to me, is the darkly sinister one who sets herself up against Edmund with her attempt to make him take a fall from grace. The developing relationship between Mary on the evil side versus Fanny and Edmund on the good side just ripples with undercurrents of Christian symbolism. No, I don't think any of us are reading into this more than Miss Austen intended because she was THAT brilliant in her handling of such a complex and convoluted plot.