You have to make the evil claim: that the Spanish invaders were seen as a better alternative to the indigenous Aztecs by at least a decent majority of the population (and then, later, the Inca) (maybe this even explains the final end of the Maya)
Which is both likely true and morally against what we currently seem to believe.
Maybe I should not have used the word foreigner, but my comment/question is not why did they prefer ally with the spanish on a moral point of view but why did they believe the spanish were strong enough to be followed against the aztecs if not for their military superiority.
Was there a concept of "ingenuousness" for the natives? Even the Aztecs were relative newcomers to the area. They came from the north and enslaved the area. Why prefer their terror to the unknown future with the new people that appeared?