By "massacred by Cortes", surely you mean "Massacred by 700 Spanish people, and 90,000 natives who were really pissed at the Aztecs and wanted revenge". Sources indicate(if you can trust them) that the Spanish tried to stop some of the widespread atrocities that the allied natives were doing to the Aztecs, but were powerless against a group 100x their size.
Supplying the armor and weapons necessary to an army that size, knowing the likely outcome, makes Cortes culpable. And the outcome is exactly what he intended, and enabled. This could be said of any of the proxy wars fought around the world today.
But yes, not excusing a brutal tradition of sacrifice. I think my argument stands, though, that you can learn positive things from any population/civilization that has cruel actors (or cruel traditions) within it. Be it the Spaniards, despite the conquistadors, or the Aztecs, because of the warrior elite.
