In the present study, I develop an interpretation of the critical approach to morality (especially Christian morality), which Nietzsche develops in On the Genealogy of Morality. My approach is framed by his characterization of its three essays as psychological studies, and more specifically as applications of his claim that moralities are “signs” or “symptoms” of the affective states of moral agents. The relation between morality and affects envisioned here is functional, rather than expressive. The genealogical inquiries are designed to show how Christian morality is well suited to serve certain emotional needs. They reveal the role played by a particular emotional need, manifested in the affect of ressentiment and the urge for revenge. This is the need to have the world reflect one’s will, which is rooted in a special drive toward power, or toward bending the world to one’s will. Revenge is plausibly understood as aiming to bolster or restore power when it is threatened, and the adoption of the conceptual apparatus of Christian morality, including its new values, is a particular way to do so: by altering the agent’s will (her values), it alters what counts as power for her. By thus revealing how it is well suited to play such a functional role in the emotional economy of moral agents, the genealogical inquiries arouse critical suspicion toward Christian morality. The use of this moral outlook as an instrument of revenge is problematic not because it is immoral, but because it is functionally self-undermining.