I think that one point in how Cortes' assumption of power and subjugation of the Aztecs represents genocide is to examine how the action embodies some of the stages of genocide. For example, one can see the stage of polarization evident. When Cortes places Montezuma II under arrest, it represents the genocidal idea of polarization, when significant voices are silenced. This helps to pave the way for an extermination or subjugation of another group. Adding to this would be how Montezuma plead for patience amongst the Aztecs in their response to the foreign Conquistador.
Another stage of genocide that is evident is the stage of extermination. The death that resulted from the "West" interacting with the indigenous people of the region is staggering. Vast numbers of suffering and inflicted pain became the result of this interaction:
...all the houses and stockades in the lake were full of heads and corpses. It was the same in the streets and courts … we could not walk without treading on the bodies and heads of dead Indians. Indeed, the stench was so bad that no one could endure it.
In this, the stage of extermination in the genocidal process is evident in how Cortes and his followers sought to use arms and weapons in wiping out an entire group of people.