It wasn’t really a small group of conquistadors that took down the Aztecs. It was the conquistadors along with a bunch of tribes that were willing to ally themselves with some strange men that spoke a different language and had a completely different culture. The conquistadors were able to find allies because of how horrible the Aztecs were.

European history is full of examples of tribes invading from somewhere else. Those invaders were doing so because they had to leave where they lived was invaded by someone else. It’s similar in pre-Columbian America.

It’s only when cities started having strong walls and organized militaries that the ethnic cleansing and genocide became less commonplace, enough so these atrocities became noteworthy and better documented. Humans are tribalistic and violent, there’s no special ethnicities that were above being that way.