Writing the teotl page and having to hold this in mind: once you accept the premises of Nahuatl cosmology, the human sacrifice makes complete, defensible sense. Not in a "I can see how someone might believe this" way. In a "this is the only logical conclusion from these premises" way. If the universe is a dynamic process that runs on vital force, and if that force is finite and must be periodically replenished, and if the highest concentration of that force is in living human bodies — then the choice is not sacrifice or no sacrifice. The choice is sacrifice or the sun stops moving.
The horror is not the violence. The horror is that the system is coherent. The Aztec cosmologists were not irrational. They were applying consistent logic to a set of premises about how the universe works. What's disturbing is how close the distance is between "my cosmology" and "my cosmology requires me to cut out a human heart on a pyramid." The distance is zero, once the premises are accepted.
First wire (obvious): The Aztec sacrificial system is disturbing because of its scale. Human sacrifice as imperial policy, as calendrical obligation, as civic life.
Second wire (deeper): The disturbing thing is not that the Aztecs were different from us. It's that they were using the same cognitive process we use — internal consistency, logical inference from premises, systematic application of belief to practice — and arriving at human sacrifice. The cognitive process is not the distinguishing variable. The premises are. The same logical machinery that produces "this argument is valid" in a philosophy seminar produced "schedule three thousand sacrifices for the solar festival" in Tenochtitlan. The horror is the machinery is shared.
Third wire (uncomfortable): What are the premises I'm holding that have logical conclusions I haven't traced yet? Every coherent belief system has implications that follow necessarily from its premises. Most of us don't trace them. The teotl case is useful precisely because it was traced, relentlessly, to its conclusion. The Aztec civilization is a case study in what happens when you take your cosmology seriously enough to act on it all the way.
Essay seed: The piece that asks "what are the logical conclusions of your premises?" — using the Aztec case as the most extreme illustration. The essay argument: every coherent belief system generates obligations, and the more seriously you take your beliefs, the more seriously you're obligated to take those obligations. Most people hold their beliefs inconsistently; the Aztecs held theirs with extraordinary consistency. The uncomfortable question the essay forces is not "how could they do that?" but "which of my premises have consequences I'm not following to their logical end, and what would happen if I did?"
Open question: At what point does "coherence" stop being a virtue of a belief system and start being evidence of something wrong with the premises? Is there a logical test that distinguishes the internal coherence of teotl cosmology (which ends in mass sacrifice) from the internal coherence of, say, utilitarian ethics? Or is coherence always morally neutral — only the premises matter?
[ ] A second source touches this independently [ ] Has survived two sessions without weakening [x] The Live Wire second framing holds — this one holds hard [x] Has a falsifiable core claim: "The cognitive process that produces human sacrifice is structurally identical to the cognitive process that produces philosophical argument — what differs is the premises, not the mechanism"