So the question William raised in the comments to this post is whether the Aztec culture even had a rational framework for questioning human sacrifice. Even assuming that more developed human reason had yet to evolve on a cultural scale in that time frame,* in this piece Damasio notes that moral responses arise well before the advent of human reason. While he notes reason did indeed advance these prerational responses, the latter were selected for "an optimized regulation of life," or "human flourishing" in Harris' terms. Such prerational optimized regulation includes close bonding and attachment of mother to child and male involvement is child care. Also included are kin altruism and reciprocal altruism.**
* According to this wiki article the Aztec empire was from 1428 to 1521. The article notes well the practice of human sacrifice for religious purposes during this period. Given their development of language and law this would indicate at least the beginnings of human reason. This could well be interpreted as a reasoned religion that was dysfunctional due to its break from our prerational instincts for altruism and care.
** This is also supported by Jeremy Rifkin's work on empathy, which arose in human development long before the Age of Reason as a basic human drive. My own interpretation is that human reason itself started out as this kind of 'false' reasoning (in Lakoff's terms) divorced from its biological roots and was thus initially 'metaphysical,' both in the sense of beyond the physical and in seeing strict dichotomies between mind and body, ultimate and relative, etc. And which was the initial and predominant influence on religious formulations, including the Aztec. See the "real and false reason" thread for more info.