The Aztecs in particular were kind of uniquely terrible, both for their own citizens and for every oppressed pseudo-vassal-state around them. It's one of those weird accidents of history that Spanish colonizers were able to step into the power vacuum after the fall of Tenochtitlan and have at least some people genuinely think 'yes, this is better than the last boss'.
Cortez came to Tenochtitlan as an 'ambassador' at the head of an army of 200,000 angry neighbors of the Aztecs, who had realized pretty fast that even a few of these 'gun' things would be really useful for cracking the city's structural resistance to sieges.
