The major bloodbaths took place further south with the overthrow and slaughter of the Inca and Aztec empires -- with populations of 12 million and 5 million, respectively.
> It's the UCL group's estimate that 60 million people were living across the Americas at the end of the 15th Century (about 10% of the world's total population), and that this was reduced to just five or six million within a hundred years.
The only thing that was that early, that rapid, and that devastating was the diseases introduced by early European visitors that preceded their colonization by decades and leagues.
The New Spain conquest (current Mexico) was done with the help of natives that hated Aztecs. Even with the help of gunpowder it would have been almost impossible to achieve that body count. Thus, the Spanish Crown wanted the help of the Catholic Church, so it promises keep the population to christianize them.
What's your point?