An advanced civilisation if there was one, Aztecs lived in Central America from IXth Century to XVth Century (when their decline was essentially precipitated by their encounter with the Spanish conquistadores). Many facets of this civilisation stay unknown principally because they were wiped out wy the first European colonizers.

But one aspect stayed deep in the minds because it was an immediate shock to those who were meeting it for the first time: Aztecs practiced extensive human sacrifice when Cortés arrived in America. Rare are the Europeans who observed it first-hand, because it stopped quickly then, but many traces are left for the historian.

Generally, religious reasons (“gods are asking for peace brought to them through human bloodshed in their honor”) are shown as the main driving factor. I found a group of articles quite intersting because they discuss the vertus and weaknesses of another theory: That Aztecs needed to enrich their meals with meat that could only be obtained from cannibalism.

• The Enigma of Aztec Sacrifice, by Michael Harner (April 1977)
• Aztec Cannibalism: An Ecological Necessity? by Bernard R. Ortiz de Montellano (May 1978)

I can’t guarantee the quality of the scientific arguments by myself, but -for the reader with a strong heart- the LatinAmericanStudies.org web site contains historical data about the Aztecs grouped in a quite enthralling scientific package.

Additional references:

1. Wikipedia article on the Aztecs.
2. Photos about the Jaguar and Ocelotl, the jaguar Aztec god.