I’ll try and cover a few of your specific points, starting with the fact Apocalypto did not intend to portray the Aztecs, but the Maya. The film does (poorly) mash in some aspects of Aztec sacrifice, if only to further its goal of being colonialist apologia and torture porn. Fortunately, the sheer awfulness of the movie makes it a good jumping off point to talk about actual practices of sacrifice.
To start with, there were slaves in the Aztec world and a portion of them did come from slave raids. The whole notion of actual warriors going out to get slaves for sacrifices, however, is a bit ridiculous. While slaves would sometimes be used for sacrifices in particular circumstances, the majority of sacrifices stemmed from war captives. Taking a captive was considered a rite of passage for a young warrior and a requirement for military and social advancement. Note, however, that simply snatching up some schmuck from a podunk village was not a standard practice; the expectation was taking a captive in battle. Also, later in the Imperial phase of the Aztecs, certain opponents became so little regarded that even taking several of them in battle earned little more than a shrug, as this passage from Sahagun illustrates:
And if six, or seven, or ten Huaxtecs, or barbarians, were taken, he gained thereby no renown.
Conversely, taking captive from more formidable opponents, such as those from Atlixco and Huexotzinco (which were coincidentally in the hard-fought borderland with Tlaxcala), earned great acclaim. So the notion of Aztec warriors raiding villages too small to apparently even have maize fields does not make sense.
Once captives were taken there are some scant mentions of using cages. From the same book of Sahagun:
And there in battle was when captives were taken. When it had come to pass that they went against and conquered the city, then the captives were counted, there, in wooden cages: how many had been taken by Tenochtitlan, how many by Tlatilulco…
So using cages was a real thing, but there’s no indication they were anything but temporary measures. For instance, they were also used during the sale of slaves, or when holding prisoners during trials. Captives were not simply rounded up and kept indefinitely like cattle in pens. Instead, captives were treated, well, like slaves, to be housed by their captors until the time of their sacrifice.
Were those sarifices a public spectacle? Well, yes and no. Many of the sacrifices were public events, and some specifically so in a way that demonstrated the power of the Aztec state. Rulers and dignitaries of foreign, even enemy, nations would be invited to witness these displays as a form a intimidation.Apocalypto portrays these sorts of events as a wild bacchanal of primitives gyrating in a wild, unhinged frenzy. In fact, if we turn to sources like Duran or Sahagun, we see that even the most public and bloody ceremonies were highly regimented rituals of specific songs, dances, offerings, and adornments, each with its own meaning. There was an aspect of spectacle, but ultimately these were religious rites.
We can see the combination of somber and spectacle in accounts of the “gladiatorial” sacrifice which took place during Tlacaxipehualiztli. After weeks of preliminary rituals, captors would bring their captives to a particular calmecac, Yopico, in the Sacred Precinct. There the captor would lead his captive up to a raised platform upon which lay a large heavy stone. Tied to the stone and armed with a macuahuitl whose blades were feathers, the captive would face up to four elite warriors (and a fifth left-handed one if he managed to “defeat” the four), but would ultimately be sacrificed on that stone once he faltered.
So there’s certainly some spectacle there and the whole notion of “gladiatorial” combat evokes the Colosseum, but there’s some substantial differences. For one, there’s some dispute as to the “public-ness” of this event. Sahagun mentions no one but the priests and the warriors, which does not preclude the presence of others. Duran, meanwhile, says the “entire city was present,” although the location of the particular calmecac where the combat took place was a smaller building off in one corner of the Sacred Precinct, which present problems for mass viewing.
More importantly though, the intentions were different. Even this particular sacrifice, which was among the largest (dozens are mentioned as sacrificed over the course of a day) and the combat making it among the most dramatic, the core aim was not to provide tititallation, but serve both as a sort of graduation ceremony for warriors who had taken a captive and also a way of providing “sustenance” to the gods. On that latter part, just as important as the actual combat was the captor taking the blood of his sacrifice, collected by the priests in a bowl, and going from idol to idol having them take a “drink” from the bowl. Considering the symbolic impetus of Aztec warfare was to engage in battle in order to “feed” the gods, this act not only completed that divine onus, but the entire gladiatorial spectacle re-created the process of warfare/capture/sacrifice. This was not just bread and circuses, in other words.
Speaking of bread, Tlacaxipehualiztli accounts have direct references to the consumption of human flesh, with the captive being divided up for the home and neighborhood of his captor. Famously, the captor would decline to feast on his own captive, saying:
“Shall I perchance eat my very self?” For when he took the captive, he had said: “He is as my beloved son.” And the captive had said: “He is my beloved father.”
This passage from Sahagun does end, however, by noting that the captor might partake of someone else’s captive.
As we’ve already seen with the feeding of the gods, the notion of captives as divine sustenance was an important symbolic concept, so we can’t simply see the act of consuming a captive in nutritional (or even culinary!) terms. This was the mistake Harner made in his 1977 article, “The Ecological Basis of Aztec Sacrifice,” which Marvin Harris would proclaim as having “solv[ed] the riddle of Aztec Sacrifice” in his book published the same year, Cannibals and Kings: Origins of Cultures. Actually, Harner made a number of mistakes, but the strict cultural materialist approach they used is notable for excluding any cultural explanations of societal behaviors in favor of ecological causes. So already we have to understand that Harner and Harris were using a flawed approach to Aztec sacrifice.
The other thing we have to understand is that Harner was not a Mesoamericanist and did not have a thorough understanding of the society he was proclaiming to explain. If he did have a deeper understanding he might not have made so many glaring errors in his hypothesis. To briefly sum up his position, Harner believed Aztec society was uniquely protein deficient, seeing as how it lacked the large domesticated animals of Afro-Eurasia, which was made up of empires “based on economies with domesticated herbivores providing meat or milk.” In response to this, the Aztecs turned to preying on their neighbors to meet this dietary need. Harris expands on this view and tries to blunt criticism of how many sacrifices would have been needed to feed the vast population of the Aztecs, by positing that even if only the elites were engaging in cannibalism, that would be enough to sustain this “cannibal empire.”
Unfortunately for Harner and Harris, the foundation of their argument was flawed, because they were ultimately viewing the Aztecs through an ethnocentric lens. They focus, almost exclusively, on dogs and turkeys as sources of protein, with lesser mentions of waterfowl, fish, and wild game like deer and rabbits. Both disparage the use of tecuitlatl, the spirulina algae that was collected form the lake and pressed into cake, which is like disparaging McDonalds – it may be a food of subsistence for some, but that doesn’t mean it wasn’t eaten by peasant and presidents alike. Indeed, their approach basically glosses over the innumerable foodstuffs eaten in Mesoamerica that are strange to the Western palate. Even as Harner quotes Cook and Borah saying “just about everything edible was eaten,” he refocuses on dogs, turkeys, and men.
Ortiz de Montellano, in his 1978 article, “Aztec Cannibalism: An Ecological Necessity?” to Harner to task by listing all of the various other protein sources in the Aztec diet which are attested to in the literature, which included iguanas, snakes, frogs, and salamanders, as well as various insects and insect larva. He further notes that Harner ignores the traditional Aztec staples like amaranth and chia, the former of which is a grain high in protein and the latter a seed with good protein and fat content. The fat content is key, as Harner and Harris see this as an important key to understanding Aztec cannibalism: it wasn’t just protein deficiency, but also fat deficiency. This ignores not only chia, but also crops like avocados. Ortiz de Montellano further notes that Harner does not address the fact that the core Aztec cities were the recipient of tribute bringing innumerable bushels of maize, amaranth, and chia to the populace, before noting that humans are actually a very inefficient source of protein and that the number of sacrifices required simply do not add up.
Increasing the number of sacrifices per annum is thus vital to Harner and Harris. Unfortunately, they rely on some dodgy numbers regarding Aztec sacrifice. Harner starts by taking an estimate from Cook of 15K sacrifices per year throughout the Aztec empire, on the basis of a population of 2M. He then revises this number upward, citing personal communication with Cook. The end result is Harner posits 250K sacrifices a year in a population of 25M. The problem is that we have no reason to think that an increase in population would lead to a proportional increase in sacrifices, yet this is essential to Harner’s idea of sacrifice as ecologically driven. Under his cultural materialist model where sacrifice is intrinsically tied to the dietary needs of the population, they must be proportional, but he is essentially pulling numbers from nowhere.
The problem is that our actual reports of captives taken do not support those numbers, though they are scanty and far between. Adding up the numbers of sacrifices mention by Sahagun in his book on ceremonies likewise does not add up to the numbers Harner needs, but we can likewise not rule out additional sacrifices going unmentioned. The truth is we do not have good numbers for how many people were sacrificed. We do not, however, have any reason to believe that the numbers of sacrifices in Tenochtitlan, which was the center of an unprecedented religious focus on sacrifice, would be replicated throughout other regions of Mesoamerica, even those areas subject to the Aztecs. As Brumfiel points out in her chapter “Figurines and the Aztec State: Testing the Effectiveness of Ideological Domination,” outside of the central Aztec cities, we see a markedly different archaeological profile of religious figures, which she suggests points towards a highly militaristic and sacrifice-driven state cult of war gods, which gave way to a more traditional model of agricultural deities and less sacrifice-focused practices in the countryside.
The end result is that we have no reason to accept Harner’s proposition that 1% of the total population of Mesoamerica was sacrificed every year, particularly since the late Postclassic is marked by a substantial increase in population as the same time he and Harris are proposing a life of cannibalistic subsistence. The Aztecs certainly focused on and increased the rate and importance of human sacrifice beyond what had been previously seen in Mesoamerica. None of the actual ecological or dietary data suggest their society needed to rely on cannibalism, and the focus on that aspect of their society tends to overlook other ways in which the Aztecs were a highly organized and functional pre-modern agricultural society, whose population boomed and whose marketplaces were stocked with non-people foodstuffs.
Aztec sacrifice was a complicated and, to the modern Western view, bizarre practice, but it was not the sole aspect of Aztec society. It was, however, neither as alien to practices found in Afro-Eurasia, nor a perfect analogy to them. It wasn’t sadists fattening up captives in cages; the practice had a logic to it. Aztec sacrificed evolved from a general pattern of sacrifice in Mesoamerica going back millennia, and the religious and social aspects of Aztec sacrifice were adapted to the realities of their time.