Because the natives joined in with them. The Aztecs were hated and ruled through military subjugation. They had a strict class system where the warriors were always trying to expand the empire and held most of the power. They required tributary and glory for the warriors (Aztecs started out as mercenaries). They'd invade their neighbors and bring in captives for human sacrifices... During one of their big holidays they "sacrificed" over 20,000 people by ripping open their chests with rudimentary knives.

And they also ritualistically tortured, killed and cannibalized children during the dry season. https://www.eiu.edu/historia/Thoele.pdf

That was extreme and insane to most of the native inhabitants. And it was even more outrageous to Europeans (or Asians). The Spanish weren't hero's and they exploited the natives and unwittingly spread disease but the Aztec civilization had to go and it died just as much from internal revolt than from the small amounts of European men that Cortes commanded. I have more respect for the Pueblo's than I do for the Aztecs. They survived harsher environments and had impressive self-taught farming and structural engineering skills.

This is not an apologist perspective, my heart weeps for the knowledge that was lost. But from the perspective of the Spanish conquistadors, all of Aztec culture looked like devil worship. Bear in mind these were Catholics who were suddenly immersed in a culture that regularly performed ritual human sacrifice. So they buried their statues and burned their books. It's tragic, but not all that surprising, really.

I think that's how they justified it, but all the looting and slave labor are what required the justification.

The book "Why Nations Fail" spends a while discussing the Spanish approach to the Americas, and "Extractive" doesn't even begin to cover it.

You do realize that the Aztecs also practiced slavery as well.

That in no way justifies the behavior of the Spanish at the time.

Religion

They were getting fantastically rich.

It would have been horrifying to witness mutilations and sacrifices.

https://en.wikipedia.org/wiki/Human_sacrifice_in_Aztec_cultu...

Would it have been more horrifying than seeing public executions or bodies mutilated through castration or circumcision in Europe?

False reporting by the people who were on the ground, people with the perverse incentives of fame and fortune.