The Spanish primarily won against the Aztecs due to alliances with rival tribes and, you guessed it, plagues. Some experts have cited a figure as high as 90% death in the "New World (including North, Central, and South America" from old world diseases.

Also, the Aztecs used a complex irrigation system and Tenochtitlan was one of the largest cities in the world at the time with a population of around 200k. They were not a "backward" or "undeveloped" people, this was a highly developed urban society. The main differences were the Aztecs had not developed significant animal husbandry or metalworking since they had no need of it at the time. If you want to talk about barbaric religious practices tell me about the Spanish Inquisition or the Crusader sieges and what they did to the civilians while their Muslim foes showed mercy.

But sure, go ahead and glorify the genocidal conquistadors seeking El Dorado to fund more European wars. The same folks who set up a racial hierarchy on the continent that refuses to die to this day.