The Aztecs sacrificed (read murdered) 40,000 Indians of subjugated tribes every year.
How could Cortes, with less than 500 men have conquered the Aztec Empire, whose capital, Tenochtitlan, had a population of 1 million? He was able to pull it off because the Indians that were subjugated by the Aztecs joined the forces of Cortes.
I'm referring very specifically to the island people wiped out by Columbus. All we know about them comes from Columbus' and his crews' writings, which may not be accurate.
Remember, Columbus did not discover what is now the USA, I think he didn't even discover Mexico (at least on the first voyage, I don't know about the rest), he just landed on Hispaniola and maybe a few other islands.