We aren’t talking about modern times here-cultural relativism didn’t exist as a term or even a concept. It does not even apply to those times-the political and cultural goings-on were not the same as now-nobody’s culture was even considered-only taking other people’s stuff by military conquest, getting some slaves, territory and turning a nice profit by exploiting the natural resources and shipping them over to Europe to sell, etc, etc-religious conversion was just another tool to do that-apples and oranges.
Western civilization, Asian civilization, etc was not above genocide for fun and profit back then-especially the Spaniards-they’d already been torturing and killing anyone who rebelled in Spain since they became part of the Roman Empire over 1000 years before.
The Aztecs weren’t all annihilated, either-the smart ones converted, Cortez’s mistress founded a convent, and many people went back to where they came from in the 1st place a few hundred years before-to the American SW and their Apache cousins-they share DNA and the same language group...