Resentment (or, to be fancy, ressentiment—signifying the technical Nietzschean definition for the term), is a horribly special emotion. If, for Heidegger, authentic boredom and anxiety fundamentally attunes us to the world, then resentment leads us eternally astray.
Resentment is anti-life. It denials rather than affirms appearance. It is not hatred, anger, or envy, and differs from them fundamentally as fear differs from anxiety in that resentment directs not at a particular thing, but at Being itself, at everything that appears, exists, and thrives (and hence, resentment is also reflexive, directed at oneself). I can envy a person, angry at society, or hate the world, but no matter how multitudinous an object my intentional state is directed towards, it never directs indiscriminately. Resentment is different, and herein lies its insidiousness.
Resentment arises in a threefold process. First, one contains one’s feelings, harbors it within oneself. Then, one pushes it down, represses the hatred/envy/anger (etc.), erases it from memory. At last, because repression is just the return of the repressed, the ontic feeling is sublimated into an ontological resentment. What is directed at some thing is directed, now, at being at large.
But directing at being is basically directing at nothing at all, therefore the paradigm of intentional “directing”/”aboutness” is not a satisfactory description of what is going on in resentment. To be more precise, resentment permeates the world, envelopes it. Rather than eliciting a force (Austin), resentment always-already is a field. As a field, it becomes the natural way of being—like a myopic, the distortion, the blur, is original, and one needs to put on a glasses, making constant effort to fight resentment, in order to recognize it. Because of this, there is a perverse sneakiness in resentment, though “his soul squints; his spirit loves hiding places, secret paths and back doors” (Nietzsche), the hiding of resentment lies in the fact that it is under broad daylight all along. Therefore, the resenting person may not know that he is resenting, but sees his actions as natural.
Resentment, though, does not manifest itself by enacting crimes (suicide bombing, arsoning, mass shooting, etc.). Expressed so directly, it becomes one of the prior feelings before authentic repression. Instead, ressentiment operates by playing along with the rules of the game, corrupting it and subverting it, so that the universe itself becomes twisted and poisoned as much as the resenting individual. The classic study of the reversal comes from Nietzsche, who saw in the transitioning from ancient to modern morality resentment’s championing of a slave morality over the master morality, the weak over the strong. In doing so, it makes everything false, despicable, and imparts its own hatred of life onto the generations to come. It is a crime which hides something at the foundation of the world.
My source: On the Genealogy of Morals (Anti-Christian), and Ressentiment (Max Scheler; an interesting defense of Christianity against Nietzsche whilst embracing the concept).