Dead from smallpox and measles. This is the inevitable result when you are mixing populations from bioregions which had been geographically separated since the breakup of Pangaia. This is extremely well explained in this article from the Smithsonian-related Discover magazine, The Arrow of Disease.
The barbarous, slavery-based societies of the Incas, the Mayas and the Aztecs collided with the only slightly less-barbarous gold-seeking Conquistadores, imperfectly restrained by the law-and-civilization-generating Catholic Church, which served the purpose of giving them bad consciences by telling them that slavery was wrong and that the native people had a right to Life, Liberty, and Property.
The excesses of the barely-baptized Conquistdor class, however, do not include the crime of genocide, since disease exterminated New World populations rapidly before anybody had the barest conception of its vectors and causes. There is literally nobody to blame for that, just as there is nobody to blame for the Black Death which halved the population of Europe in the 14th century, as a result of contact with microbes carried in by the incursions of peoples of Central Asia to the coast of Dalmatia, and then carried by sailors (and ship rats) to all the port cities of Europe.
Pre-Columbian Mesoamerica had its own slavery, genocide, and oligarchic priestcraft. The big difference, when the Spanish came, is that the Church condemned slavery and genocide, but had the power only to mitigate it and not to abolish it.
You might try learning a bit about the Catholic Church's early and persistent defense of Indian rights, including the writings of Vitoria, a distinguished professor of theology at the University of Salamanca between 1526 and 1546, Domingo de Soto, Francisco Suarez and Bartolome de Las Casas.
Sure I believe that!