Most scholars of Pre-Columbian civilization see human sacrifice among the Aztecs as a part of the long cultural tradition of human sacrifice in Mesoamerica stretching back thousands of years.
Various extant sources describe how the Aztecs sacrificed human victims on each of their eighteen festivities, one festivity for each of their 20-day month, every year…
Why did they sacrifice?
In the Aztec “Legend of the Five Suns”, all the gods sacrificed themselves so that mankind could live. That act was reciprocated, on the part of the Aztec through human sacrifice as a way to honor and recognized that sacrifice.
At university I was told an interesting story that some years after the Spanish conquest of Mexico, (February 1519 – August 13, 1521) a group of Franciscans confronted some of the remaining Aztec priesthood and demanded, under threat of death, that they either desist from this “traditional practice.” or else.
The bewildered Aztec priests couldn’t understand why anyone would want to interfere with the important work they were doing (or believed they were doing). They defended themselves as follows:
“Life is because of the gods; with their sacrifice they gave us life…. They produce our sustenance… which nourishes life.”
It is speculated that the Aztec priests were referring to was a central Mesoamerican belief: that a great, on-going sacrifice sustains the Universe. Everything is tonacayotl: the “spiritual flesh-hood” on earth. Everything —earth, crops, moon, stars and people— springs from the severed or buried bodies, fingers, blood or the heads of the sacrificed gods.
Indeed humanity itself is macehualli, “those deserved and brought back to life through penance”.
Needless to say a strong sense of indebtedness was connected with this worldview that connectedness was paid for in blood. Indeed, nextlahualli (debt-payment) was a commonly used metaphor for human sacrifice, and, as Bernardino de Sahagún reported, it was said that the victim was someone who “gave his service”.
See related video: Aztec Sacrifice https://www.youtube.com/watch?v=hQKJRnPpIxw