Zarathustra calls those who preach democracy, equality, and justice "tarantulas": secretly, they spread the poison of revenge. By preaching equality, they seek to avenge themselves on all those who are not their equals. Life thrives on conflict and self-overcoming. If we were to make everyone equal, how could we strive for the overman?
Nietzsche's contempt for "the rabble" and for egalitarian sentiments goes deeper than mere snobbery or elitism. In ##The Genealogy of Morals##, he draws a sharp distinction between the "master morality" of the ancient aristocracy that we find for instance in ancient Greece, and the "slave morality" that developed among the lower classes and the priestly caste. While his attitude is far more complex than simply approval of master morality and disapproval of slave morality, there is a great deal about slave morality, as expressed in Christianity and democracy, that he finds contemptible.
The weak and powerless establish slave morality as vengeance upon their aristocratic masters. We find the concept of ressentiment, or resentment (Nietzsche uses the French word), at the bottom of Nietzsche's account of slave morality. The weak resent the power of their masters, and resent even more their own inability to enact vengeance upon their masters. Because they are unable to strike back in any substantial way in this life, the weak invent the idea of an afterlife and of divine justice, which will avenge them after death. Thus, divine justice is the invention of a people too weak to secure justice for themselves.
The slave class also invented the concept of "evil," which Nietzsche cites as one of humankind's greatest inventions. The aristocratic masters, and everything about them—wealth, health, happiness, strength, vigor—were considered "evil" and contemptible. By contrast, then, the slaves identified the notion of "good" with everything that these masters were not: poor, unhappy, sick, weak, mediocre—in short, they identified themselves as "good." This new slave morality was a complete reversal of the older, master morality.
Nietzsche understandably identifies Christianity and democracy with slave morality. The biblical Sermon on the Mount (Matthew: 5–7) is one of the clearest examples of Christianity as slave morality. In it, Jesus commends a life of meekness and poverty, devoid of earthly riches. As for democracy, it is based on notions of equality and justice that Nietzsche would probably view as being rooted in slave morality. As we saw, Nietzsche considers the idea of justice to be the invention of those who cannot secure justice on their own. Democracy ensures that the weak do not have to suffer the abuses of the strong and that the strong cannot oppress the weak. At least, that's what democracy is supposed to do.
Nietzsche is not entirely opposed to slave morality. In particular, he admires the creativity and the sublimating power of people who are capable of reversing an entire system of morality. However, he strongly opposes the spirit of ressentiment as petty and contrary to life, as it leads to the conclusion that life is something that must be suffered and that justice and happiness are to be found after this life. This spirit of ressentiment is fundamental to slave morality, and Christianity and democracy are doubly damned, first for basing their morality on ressentiment, and second, to be hypocritical enough to deny this fact. Neitzsche would say that modern Christians and democrats even lack the creativity of the originators of that slave morality. Today, they are just uncreatively perpetuating a tradition.