Why did the Aztecs practise human sacrifice, and how many people really died?

The Aztecs practised human sacrifice because they believed in a cosmic debt. In their legend of the Five Suns, all the gods sacrificed themselves so that mankind could live. Aztec priests later told Franciscan friars that life exists because of the gods, whose sacrifice gave people life and produced their sustenance. In that view, sacrificing people was a way of repaying a debt, and the Nahuatl word nextlahualli, debt-payment, was a common metaphor for it.

The practice was not new to the Aztecs, and not unique to them: human sacrifice was common across Mesoamerica. What set the Aztec practice apart from the Maya, according to scholars, was how embedded it was in everyday life. The most common form was heart extraction, and the Aztecs believed the heart was a fragment of the Sun's heat. Many victims were captured in ritual battles called flower wars, whose main goal was to take enemies alive.

How many people died is where scholars disagree. Later sources claimed about 80,400 victims at one temple dedication in 1487, but the historian Ross Hassig called that an exaggeration, and an Aztec codex records old Aztecs giving a figure of about 4,000. Estimates for a whole year range from a few hundred in Tenochtitlan (Robert William Martin) to 1,000 to 20,000 across the empire (Caroline Dodds Pennock). Almost all the written accounts come from Spanish and Indigenous writers after the conquest, so the numbers are hard to trust. Archaeology shows the practice was real: 603 skulls have been found at one skull rack beside the Great Temple.