Cortes could not comprehend worship of several gods and appeasement of them through human sacrifices. (Although he didn’t seem to have any problem burning people at the stake or accepting gifts of native maidens and distributing them among his officers). There was only one faith; all others must be destroyed.
Montezuma could not comprehend temples in which a cross replaced braziers on which human hearts, still quivering after being ripped from some unfortunate's chest, were roasted for the gods.
There could be no compromise. No room for more than one belief. The Aztecs lost, and world changed -- in some ways. More than 500 years later, however, we still haven’t learned to tolerate completely other religions, or other cultures. (Also, we’re still having some problems in respecting women, as demonstrated by Arnold Slimenator, Dominique (The Grand Seducer) Strauss-Kahn, John Edwards and dozens of other “leaders.”)
Parts of the Muslim world still argue over how to relate to non-believers. Jews and Arabs continue the dialogue of hatred. The Roman Catholic Church, trying to stay away from the larger issues, has just finished 29 years of fiddling with its Mass missal. One of the big changes is that Jesus as “one in being with the father,” (old version), now is referred to as “consubstantial with the Father.”
No wonder there is a trend to spirituality outside organized religion. Religion, like party politics, often puts too much emphasis on things that distract from the goal of achieving greater common good. Both politics and religion are needed by many, but both need to be refocussed on things that really matter.
Cortes and Montezuma arguably couldn't help being the way they were. Five hundred years, and much enlightenment later, there is no excuse for us.