Character Analysis
The Marquise de Merteuil is a wealthy widow who's even more heartless than Valmont, if that's possible. She's managed to cultivate a reputation as a virtuous woman while taking a series of lovers who she toys with and tosses out. Her two pleasures in this novel are getting revenge on her ex by ruining his virginal fiancée, and helping Valmont destroy the lovely Madame de Tourvel. And when she's done with that, she tries to destroy Valmont.
The Marquise is used to getting what she wants. When she writes to Valmont demanding his assistance in getting revenge on Gercourt, she doesn't ask politely.
Add to these recommendations that it is I who make them, and you have only to thank me and obey. (1.2.4)
When Valmont, at the end of the novel, gives her an ultimatum either to be his lover or his enemy she doesn't hesitate to declare war (4.153.6). The Marquise knows who she is and what she wants. She doesn't hesitate and she doesn't fool around. When she wants to make a kill, she goes for the quick mortal wound. She works mostly behind the scenes, collecting and dispensing information to further her plots. Unscrupulous is the best word to describe her, and she's spent a lifetime learning to be that way.
She's got way too much time on her hands, as did most wealthy women of her day, and to avoid being bored to death she amuses herself by ruining the lives of other people.
You Don't Own Me
Although the Marquise is every bit as manipulative and ruthless as Valmont, her motivations are different. As a woman, even a wealthy one, her social standing limits her. Men have it easy, she thinks:
And where, after all, is the achievement of yours that I have not a thousand times surpassed? You have seduced, even ruined a great any women; but what difficulty did you have in doing so? (2.81.10)
For you men, defeat means only one victory the less. In this unequal contest we [women] are lucky not to lose, you unlucky when you do not win. (2.81.12)
Valmont might have a sense of entitlement because he's a rich male aristocrat, but as a woman, the Marquise has had to carefully develop skills to protect herself from people like him. In a long letter to him, she describes how she's carefully invented herself as someone who would never be taken advantage of.
As a girl, "condemned by my status to silence and inaction," (2.81.24) she made it a point to watch people carefully and learn to lie and even control her facial expressions and tone of voice. By the time she was fifteen, she could read people better than most politicians. She developed a detachment that allowed her to learn from her experiences rather than have any feelings about them. Even on her wedding night:
I awaited the moment of enlightenment with confidence, and had to remind myself to show embarrassment and fear. The first night, which is generally thought of as "cruel" or "sweet," offered me only a further opportunity for experience. I took exact account of pains and pleasures, regarding my sensations simply as facts to be collected and meditated upon. (2.81.32)
Once her husband died, she made a show of being a grieving widow for a while, then embarked on a career of attracting and discarding a series of lovers, with no concerns except for her own pleasure and control. She's kind of the female version of Valmont.
She explains to Valmont why she ever remarried:
Do you know, Vicomte, why I never married again? It was certainly not for lack of advantageous matches: it was solely so that no one should have the right to object to anything I might do. It was not even for fear that I might no longer be able to have my way, for I should always have succeeded in that in the end: but I should have found it irksome if anyone had had so much as a right to complain. In short, I wished to lie only when I wanted to, not when I had to. And here you are, writing me the most connubial letters possible! (4.152.2)
The Marquise, skilled as she is in reading people, definitely has Valmont's number. When she wants to wound him, she gets him where he's most vulnerable: his confidence in his "conquests."
Tell me then, languishing lover: the women you have had—do you imagine you violated them? Don't you know that however willing, however eager we are to give ourselves, we must nonetheless have an excuse? And is there any more convenient than an appearance of yielding to force? […] which keeps up an appearance of taking by storm even what we are quite prepared to surrender. (1.10.1)
The Marquise has obviously learned how to make men think they have the upper hand and she teases Valmont for thinking he's the one in control.
Mean Girl
Like Valmont, the Marquise de Merteuil has a vicious and heartless streak. Well, maybe more than a streak. Here's what she says about her latest paramour:
I am sure that if I had the good sense to leave him now he would be in despair, and nothing amuses me so much as a lover's despair. He would call me 'false', a word that has always given me pleasure. There is nothing more welcome to a woman's ear, excepting 'cruel', which, however, one must take more pains to deserve. Seriously, I am going to consider breaking with him. See what you have brought about! I lay it on your conscience! (1.5.5)
Without an ounce of humanity, Madame de Merteuil embarks on her plan to have young Cécile debauched. It's all part of the game. She's proud of her cruelty; it took years to perfect. If the Marquise seems even more evil than Valmont, it's for that reason. Some of Valmont's cruelty is absolutely intentional, but we get the impression at times that he was just born that way, thoughtless and self-absorbed. A lot of his abuse of women is a function of sexual desire. With the Marquise, it's all planned, all the time. She has a Ph.D. in perfidy.
Jealousy
For all her self-control, the Marquise seems to respond very badly when she starts to suspect that Valmont is actually falling hard for Madame de Tourvel. He's ignoring the Marquise—how dare he? She does her best to humiliate him:
Ah, Vicomte, Vicomte! You are teaching me not to judge men by their successes. Soon we shall say of you: 'He was once a man of mark.' (3.106.3)
It's hard to know, the Marquise being the Marquise, how much of her jealously is real and how much is just a strategy to draw Valmont in. She's not even really upset about his feelings for Madame de Tourvel; she's not in love with Valmont. It's more the intolerable implication that she's a person less worthy of Valmont's attention. Everything has been a game to her up until now, but she sees Valmont taking something seriously and she can't stand it. Her final act is to persuade Valmont to dump Madame de Tourvel, just to see if she can. Heartless.
Mistress of Deception
Having created herself from scratch, deception comes very easily to the Marquise. It's her default mode. Every letter she writes has an ulterior motive. She never expresses anything that isn't designed to achieve one of her personal objectives. She manages, at the same time, to get Cécile to confide in her while planning her debauchment at the hands of Valmont; to write to Cécile's mother as her "dear and good friend" (if she only knew…) to discourage her from allowing Cécile to marry Danceny while seducing Danceny herself; seducing Prévan while planning to set him up for accusations of rape. She's got multiple deceptions going at once, and it's effortless.
We could go on and on, but you get the picture. Lying is her way of life, but she doesn't see it as lying; it's just what you have to do to be in control of your destiny. She's managed to convince people that she's a pious and classy lady. It's an effective mask, until she's revealed as the evil person she is and her mask literally falls away.
Merteuil's Timeline