Not for the faint of heart, but here's a look into human sacrifice and cannibalism in the Aztec Empire. Unsurprisingly, much of it comes down to geography.
In the Old World, domestication of herbivorous mammals, such as cattle, sheep, and pigs, proceeded apace with that of food plants. By about 7,200 B.C. in the New World, however, ancient hunters had completely eliminated herbivores suitable for domestication from the area anthropologists call Mesoamerica, the region of the future high civilizations of Mexico and Guatemala. Only in the Andean region and southern South America did some camel-related species, especially the llama and the alpaca, manage to survive hunters' onslaughts, and thus could be domesticated later, along with another important local herbivore, the guinea pig. In Mesoamerica, the guinea pig was not available, and the Camelidae species became extinct several thousand years before domesticated food production had to be seriously undertaken. Dogs, such as the Mexican hairless, and wildfowl, such as the turkey, had to be bred for protein. The dog, however, was a far from satisfactory solution because, as a carnivore, it competed with its breeders for animal protein.
It seems that the whole Aztec Empire was organized around cannibalism:
At first glance, this prohibition against commoners eating human flesh casts doubt on cannibalism's potential to mobilize the masses of Aztec society to engage in wars for prisoners. Actually, the prohibition was, if anything, a goad to the lower class to participate in these wars since those who single-handedly took captives several times gained the right to eat human flesh. Successful warriors became members of the Aztec elite and their descendants shared their privileges. Through the reward of flesh-eating rights to the group most in need of them, the Aztec rulers assured themselves an aggressive war machine and were able to motivate the bulk of the population, the poor, to contribute to state and upper-class maintenance through active participation in offensive military operations. Underlying the war machine's victories, and the resultant sacrifices, were the ecological extremities of the Valley of Mexico.
With an understanding of the importance of cannibalism in Aztec culture, and of the ecological reasons for its existence, some of the Aztecs' more distinctive institutions begin to make anthropological sense. For example, the old question of whether the Aztecs' political structure was or was not an "empire" can be reexamined. One part of this problem is that the Aztecs frequently withdrew from conquered territory without establishing administrative centers or garrisons. This "failure" to consolidate conquest in the Old World fashion puzzled Cortés, who asked Moctezuma to explain why he allowed the surrounded Tlaxcalans to maintain their independence. Moctezuma reportedly replied that his people could thus obtain captives for sacrifice. Since the Aztecs did not normally eat people of their own policy, which would have been socially and politically disruptive, they needed nearby "enemy" populations on whom they could prey for captives. This behavior makes sense in terms of Aztec cannibalism: from the Aztec point of view, the Tlaxcalan state was preserved as a stockyard. The Aztecs were unique among the world's states in having a cannibal empire. Understandably, they did not conform to Old World concepts of empire, based on economies with domesticated herbivores providing meat or milk.