Many folks don’t know that many mesoamerican groups fought WITH the Spaniards because groups like the Aztecs were not this peace-loving people, but were conquers themselves, and treated those people like 4th class citizens.
That's true, and Cortez found many allies against Montezuma and the Aztecs among the surrounding tribes who were tired of supplying thousands of their members for the Aztecs' periodic human sacrifices. I've read that 10,000 were sacrificed to celebrate Montezuma's ascension to power.