Alternative Character Interpretation: Milady - diabolically inspired temptress, or canny, increasingly desperate woman trying to survive (with style) after being seduced as a teenager by a dodgy priest?
Complete Monster: The mysterious, murderous Milady de Winter is one of the top agents of the Visionary VillainCardinal Richelieu in his campaign to strengthen France and free it of foreign influence, but shares none of her employer's lofty ideals. Instead, she uses her position, resources, and quasi-supernatural beauty and charisma to indulge her limitless appetites for money, power, and indiscriminate, disproportionate revenge on anyone who gets on her bad side. Ever since her humble origins as a larcenous nun, she's targeted and seduced any sufficiently-useful man she comes across, destroying those who reject her and either killing or abandoning anyone unlucky enough to become her thrall as soon as they're no longer valuable. Her steadily-escalating battle with d'Artagnan brings out all her worst excesses, as she first tries to enlist him to kill her brother-in-law for his inheritance and a young noble for apparently turning her advances down, then, after he humiliates her, repeatedly attempts to kill him with zero regard for collateral damage, endangering and killing several innocents along the way, and finally murders his Love Interest as Revenge by Proxy whilst pretending to be her dearest friend.
Magnificent Bastard: Richelieu, in spite of opposing the heroes through most of the book and being quite a ruthless bastard, ultimately comes across as strangely likable to both d'Artagnan and the readers. By the end of the book, their mutual respect is so great that Richelieu gives d'Artagnan a promotion. In later books, the Musketeers look back fondly on Richelieu as a worthy adversary with some measure of greatness. Probably inherited from the original. In the second book, Athos and D'Artagnan both mildly concede that they may have been on the wrong side, considering how badly Louis fared. Mazarin thinks he's this, but his greed and miserliness holds him back.
The point could be made that Richelieu never saw them as actual enemies, and saw them more as "Those damnable kids" since when he *does* think on them, he wishes he could win them to his side, rather then killing them outright. He merely wants to rule France through the king as his puppet, and seems to admire their boldness, even if it does stymy his plans. He even takes them as his guards in the dark of night, when they were all on the field of battle against a city full of Puritan dissenters.
You have to hand it to Milady as well - locked in a small room and guarded by someone specifically chosen as loyal and impossible to seduce, she turns him into her very means of escape and of assassinating the Duke of Buckingham.
Values Dissonance: Frequently Lampshaded by Dumas, as he often breaks the narrative to wryly note that his heroes' womanizing ways were just common practice in those days. Possibly actually meant to be a Take That against practices in his own time. This trope is notably averted when D'Artagnan seems just as troubled by Athos' murder confession as the reader is. The fact that he later turned out to be mistaken only complicates the matter further.
It's not always so deliberately Lampshaded; sometimes it's there in its regular form. Note how the narrator doesn't comment when D'Artagnan pulls a Bed Trick on Milady, which by modern standards is definitely rape. No wonder she was so pissed.
It seems the main reason for this is her being one hell of Asshole Victim.
Villain Decay: Going along with Le Vicomte de Bragelonne's main theme of how the new generation is far too Romantic and not as adventurous as the previous one, the villains suffer as well. De Wardes (son of the man d'Artagnan wounded in the first book) stirs up some romantic tension and briefly duels two people, but beyond that, is a far cry from Milady or Mordaunt. Similarly, Colbert lacks the respective cunning and manipulation of Richelieu and Mazarin.