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I think the nation/tribe that were the Spanish collaborators you're referring to was Tlaxcala which were the target of habitual Flower Wars for captuered warrior sacrifice. The pleas of the Tenochca, the residents of Tenochtitlan, fell on deaf ears to the only other major power in the region the Purépecha Empire.

While Aztecs valued human sacrifice to a great extent it was due to the benefits it's bestowed in Mixtec-Pueblo culture. It was a source of not only spiritual reverence, but also military and economic superiority. It also had non domestic function as a diplomatic tool to visiting nobles and bounty haulers / tax collectors. Not to mention it served as a form of community entertainment in a similar fashion to European public executions.

As far as saying Europeans tried to slow down to make the Americas a vassal state is a misconception. Disease wiped out an apocalyptic amount of people. Following the fall of Tenochtitlan small pox ravaged the Valley of Mexico and all along the Gulf Coast. This nearly wiped out all infrastructure and Spain tried to subjugate the rest. Hilariously trying to impose a 30% tax written in Spanish and using that as a legal justification for military actions.