What moral and religious practices of the aztecs did the spaniards find strange and contrary to christian beliefs?

I think Human sacrifices and their belief in many gods. I think many Aztecs were skeptical of monotheism. They have long believed that their gods were the ones who provide them good weathers, abundant harvest. For Spaniards, Human sacrifices is a big no considering that the Christian belief would never allow human sacrifices, rather Christians would rather do good deeds and offer a stipend as a form as a sacrifice.

The correct answers: polytheism, ritual sacrifices, advanced astronomy, calendar system which is ahead of their time

There were so many unexplainable discoveries that the Spaniards uncovered as they successfully conquered the Aztecs. One of which is probably the uncovering of the "El Dorado" myth which the Aztecs believed as real. Up to now, experts are still uncovering the truth behind about it.