This paper intends to show that Friedrich Nietzsche's approach to morality or immorality involves an attempt to see moral beliefs as a product of human psychology, rather than as a set of metaphysical truths that are somehow given to, or discoverable by, us. Nietzsche wants to replace the metaphysical (or supernatural) account of morality with a natural one, and his treatment of moral belief-systems, from the perspective of this concern, can be divided into (a) a psychological analysis of the true nature of moral action and agency, and (b) an historical/genealogical tracing of the real origins of moral values. In this paper I am going to focus on the second dimension of Nietzsche's analysis through references from his polemical texts Beyond Good and Evil, Genealogy of Morals, Gay Science, and Will to Power. I will outline Nietzsche's historical leitmotif on the morality of ressentiment or slave morality and will show how it figures as a point of departure for his revolutionary transvaluation of values, one that places a new order of morality which is disparagingly called immoralism. Next, I will discuss Nietzsche's treatment of bad conscience and posit that it involves two different stages: one is the present stage of the bad conscience as a feeling of guilt, and the other is an earlier stage. I shall argue that in order for this earlier stage to develop into the level of guilt, he needs another element, namely an indebtedness towards gods, which finds its most striking culmination in the Christian heritage of religious dogmatism. Finally, I will discuss how for Nietzsche Christianity as an ascetic ideal has promoted to preserve a declining life, i.e. a slave morality, for all of humanity.