Nietzsche suggests that the "slave revolt in morality" begins when ressentiment, or resentment, becomes a creative force. Slave morality is essentially negative and reactive, originating in a denial of everything that is different from it. It looks outward and says "No" to the antagonistic external forces that oppose and oppress it. Master morality, on the other hand, concerns itself very little with what is outside of it. The low, the "bad," is an afterthought and is noticed only as a contrast that brings out more strongly the superiority of the noble ones.
While both slave and master morality can involve distortions of the truth, master morality does so far more lightly. Nietzsche notes that almost all the ancient Greek words denoting the lower orders of society are related to variants on the word for "unhappy." The nobles saw themselves as naturally happy, and any misunderstanding rested on the contempt and distance they held from the lower orders. By contrast, the man of ressentiment distorts what he sees so as to present the noble man in as bad a light as possible, and thereby to gain reassurance.
The noble man is incapable of taking seriously all the things that fester and build in the man of ressentiment: accidents, misfortunes, enemies. In allowing resentment and hatred to grow in him, in having to rely on patience, secrets, and scheming, the man of ressentiment ultimately becomes cleverer than the noble man. This constant brooding and obsession with ones enemies begets the greatest invention of ressentiment: evil. The concept of the "evil enemy" is basic to ressentiment just as "good" is basic to the noble man. And just as the noble man develops the concept of "bad" almost as an afterthought, so is the concept of "good" created as an afterthought by the man of ressentiment to denote himself.
Nietzsche remarks on how different the concepts of "evil" and "bad" are, in spite of both being considered the opposite of "good." He explains this difference by explaining that there are two very different concepts of "good" at work: The noble man's "good" is precisely what the man of ressentiment calls "evil."
Among their own kind, noble men are respectful and subdued, but when they venture out among strangers, they become little more than uncaged beasts-- "blonde beasts," as Nietzsche calls them. "Blonde" here is a reference to lions rather than to hair color, as Nietzsche bestows this name not only on Vikings and Goths, but also on Arab and Japanese nobility. The name "barbarian" is often associated with the violence that occasionally erupts from noble people.
Contemporary wisdom would suggest some sort of progress and refinement from these "blonde beasts" to the humanity of today, but Nietzsche vehemently disagrees. The overthrow of master morality in favor of slave morality is nothing to be proud of. These barbarians may have been fearful, but they were also admirable. Today's world of ressentiment is neither: it is merely mediocre. Nietzsche characterizes the nihilism he detests in contemporary society as a weariness with humanity. We no longer fear humanity, but we also no longer have hopes for, reverence of, or affirmation of humanity. Nietzsche fears that our slave morality has rendered us insipid and dull.