Dumas sets this up fairly explicitly. Milady has the heart of a monster; Kitty is sweet and trusting. This distinction demonstrates the arbitrary nature of a rigid social system, which often rewards people who have no business being at the top. Although both women are quite beautiful, Kitty’s sweetness is a marked contrast to Milady’s ruthlessness. Food for thought: why is our hero D’Artagnan so much more interested in Milady than in Kitty?
It was striking, wasn’t it, when D’Artagnan arrived breathless at the dock with his lackey, only to find that another handsome young gentleman also recently arrived with his lackey? This kind of mirror similarity does not happen with any other young man in the novel, which makes us believe that the Comte de Wardes is meant to serve as a foil for D’Artagnan. This is further cemented by D’Artagnan’s identity theft later in the novel. He seduces Milady pretending to be Comte de Wardes, but feels an inexplicable jealousy that Milady is expressing her affection to the supposed Comte. The Comte de Wardes serves first as an actual competitor (D’Artagnan must overpower him in order to board the ship for England) and then as an imaginary competitor (Milady prefers the Comte, and D’Artagnan does everything in his power to change that).