Step aside, Lana Turner. Step aside, Marion Cotillard. Step aside, deadly Bond girls. There's a new (or, to be accurate, pretty dang old) femme fatale in town. And man is she good at the femme fatale game:
He was absorbed entirely by the sensations of the moment. Milady was no longer for him that woman of fatal intentions who had for a moment terrified him; she was an ardent, passionate mistress, abandoning herself to love which she also seemed to feel. (37.8 – 37.9)
She has various aliases—Charlotte Backson, the Comtesse de la Fère, Anne de Breuil, but her most frequent epithets throughout the book are "tigress," "demon," "panther," "lioness," and "serpent." By comparing her to animals and demons, our young heroes are deliberately downplaying her humanity in order to rationalize her eventual execution. They would never treat any "proper lady" in this fashion, so they seek to establish that her actions lie beyond the rules of established conduct.
Milady is ice cold: she's intelligent and ruthless:
To be a woman condemned to a painful and disgraceful punishment is no impediment to beauty, but it is an obstacle to the recovery of power. Like all persons of real genius, Milady knew what suited her nature and her means. Poverty was repugnant to her; degradation took away two-thirds of her greatness. Milady was only a queen while among queens. The pleasure of satisfied pride was necessary to her domination. To command inferior beings was rather a humiliation than a pleasure for her. (56.4)
Her many crimes reflect these personal qualities: as a young nun, she seduced a young priest and convinced him to steal from the church; later she seduced the son of her jailer, then married the Comte de la Fère (Athos) without revealing her criminal past; she re-married an English lord (the brother of Lord de Winter) who died under mysterious circumstances shortly thereafter; she attempted to have D’Artagnan assassinated on two separate occasions; she convinced John Felton to assassinate the Duke of Buckingham; she successfully poisoned Constance Bonacieux.
Milady is a classic dangerous beauty, and in a sense embodies men’s deepest fears. She shamelessly uses her beauty to accomplish her goals, and uses her voice in particular to lure and manipulate men. As such, her character calls to mind the sirens in The Odyssey, whose alluring voices drive men to their deaths. In a similar fashion, Milady’s beauty and seduction results in men’s lives being ruined.