The Spanish had just completed a 500 year "Reconquista" of the Iberian Peninsula in which they drove out Islam. They had a military and religious culture and tradition that those who had given their souls to a false or evil god were no longer deserving of human compassion. These were the people who encountered unimaginable horrors upon encountering the "friendly natives" and "noble savages" of the New World. The had every reason to believe that they were dealing with Satan.
The manly Spaniards ran, sword in hand, to kill the blood-soaked Aztec priests on sight.
They weren't too hard to miss ..... they were usually covered in blood. Literally.