The small number of Spaniards, regardless of their superiour weapons, could not have done this alone.
They had the willing help and support of the surrounding indigenous tribes whom the Aztecs were terrorizing, killing, and using as slaves and sacrifices.
Aside from consideration of the rest of the Spanish atrocities; in this case they did the new world a favor by eliminating the Aztecs.
The small number of Spaniards, regardless of their superiour weapons, could not have done this alone.