In The True History of the Conquest of New Spain [1] (a first-hand account of Hernán Cortés' expedition in Mexico, that brought down Monetzuma's Aztec empire) there is a passage where the author, discusses the matter of human sacrifice and cannibalism with some local priests (I think these are not Aztec, but Tlascalan priests, from one of the cities allied to Cortés).
I can't find the passage in my hard copy of the book, but, from memory, the priests basically state that offering human sacrifices to the gods is the pious and proper thing to do. Castillo ends up persuaded of this, although he continues to abhor the acts they perform, of course. These priests also practiced self-mutilation, so they had noses and fingers, etc, missing. I think the point was that they were offering parts of themselves, too, as sacrifice to their gods.
There's no doubt that human sacrifice is a barbaric custom, that was historically practiced by few societies in the same scale as it was by the Aztecs. However, it is important to understand that, to them, it was not an act of evil, or even of righteous revenge towards their enemies. It was an act of worship to their gods.
My point is that we cannot apply our own concept of morality, which is probably influenced by the dominant religion of the western world, Christianity, to the manner by which people lived their life in a society that might have, for all intends and purposes, been on another planet for most of its history. If we are to judge those people, it must be on their own terms. And on their own terms, human sacrifice and cannibalism was what their faith (and therefore, their morality) required of them.
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[1] https://en.wikipedia.org/wiki/Historia_verdadera_de_la_conqu...
[2] Bernal Díaz del Castillo, https://en.wikipedia.org/wiki/Bernal_D%C3%ADaz_del_Castillo