How did the Aztecs view sin? How does that idea explain the Conquest? What enormous change did Catholicism introduce?

The Aztecs viewed sin as something that only the gods could do. Only the gods were free, living outside the universe of fated actions, so only they could sin. That explains the Conquest, because the gods were able to betray their own worshipers. If the gods had been incapable of sin, the Conquest would not have had an explanation. Instead, the defeat of the Indian tribes was understood in light of their gods' fallibility and betrayal. Catholicism changed that view of sin by placing emphasis on the individual. Each person has the ability to sin, and thus the potential for salvation. God, who cannot sin, is thus perfect and infallible. In that viewpoint, salvation becomes personal.