Alot of Indegenous Mexicans wanted the Aztecs dead , which is why they allied with cortez

[Qwark]

There is a saying, even if there are only 2 people left on earth, one person would want another person to die. That's pretty much as true today as it was back then. Many people think these tribes where all living together peacefully, but that was rarely ever the case. Usually there was one tribe somewhere that wanted the other dead.

Also divide and conquer was the most cost efficient way for colonialism.