Power is easily corrupted.
This theme is clearly at the center of this novel. This novel seems to be a criticism of the aristocracy of pre-revolution France. Money is not enough for the aristocracy. They also want to demonstrate authority in the private lives of the people around them, just to help make them feel more powerful.
Human evil is often rooted in power.
As the saying goes, "money is the root of all evil." But really, it's the desire for power that is at the root of evil, at least within the context of this novel. Money allows certain parties to have different types of influence in the lives of others. Look at Danceny. His lower social status and lack of funds leave him at the mercy of corrupt, rich people who get bored and try to ruin peoples lives, for fun.
Romance is easily twisted into a terrible weapon that can be used by powerful people.
This novel depicts the thirst for power as a sort of sexual olympics, a kind of game between wealthy hedonists. This theme is regular in French literature, famously so in fact. But in this novel, it comes into the foreground. The actual plot structure of the novel is an attempt to demonstrate how sexual seduction ends up being a powerful tool in the social dominion of the upper class over the lower class, showing too clearly that romance leaves many vulnerable to blatant manipulation or seduction.
Social perversion harms everyone involved.
At the end of the novel, in true French fashion, everyone dies, more or less. Even the characters who survive are weakened by society. Merteuil is left ugly and disreputable, and Cécile ends up living as a dishonored pariah.