The Spaniards wiped out the Aztecs because they were brutal savages who practices human sacrifice and ritual cannibalism. (Of course the Spaniards at the same time had the Inquisition going on, to their discredit.)
"The occurrence of human sacrifice appears to have been widespread and its intentions various, ranging from communion with a god and participation in his divine life to expiation and the promotion of the Earth fertility.It seems to have been adopted by agricultural rather than by hunting or pastoral peoples. Of all the worldly manifestations of the life-force,the human undoubtedly impressed men as the most valuable and thus the most potent and efficacious as an oblation. Thus, in Mexico the belief that the sun needed human nourishment led to sacrifice in which as many as 20,000victims perished annually in the Aztec and Nahua calendrical maize ritual in the 14th century AD."
ENCYCLOPEDIA BRITANNICA 1975 EDITION. VOL. 14 P-131
Topic: YOUNG AZTEC WARRIORS
"Each boy had to retain his telltale tuft of hair until he had participated in the capture of a prisoner.His first experience on the battlefield was limited to carrying a warrior's shield and observing the action, but his second required that he participate, with as many as five of his fellow novices, in taking alive a foe. The captive was then taken to those men in charge of sacrifice, who killed him. The body was divided up among the boys for their ritualistic consumption: The right thigh and torso went to the youngster who had behaved most heroically; the left thigh went to the second bravest youth; the right upper arm to the third - and so on until no part was left."
AZTECS: REIGN OF BLOOD & SPLENDOR TIME-LIFE BOOKS,ALEXANDRIA,VA:1992 PAGE-88.