Hernan Cortes was responsible for the conquest and the destruction of the Aztec Empire. Whether this was a positive or negative event largely depends on perspective. For Spain, this conquest resulted in a massive territorial gain in the New World, one which considerably enriched the empire and opened the way for further conquests in Central America. Plundering the Aztecs and people like them was a major source of revenue for Spain (and, one must add, Cortes himself.) Even this was not an unqualified success: Cortes was viewed by the Spanish crown as a sort of "loose cannon," having disobeyed orders in going to the Aztec capital of Tenochtitlan. He was an example of the difficulty in reining in the ambitious and ruthless conquistadores, many of whom did not really have the interests of the Empire at heart.
From the Aztec perspective, of course, this was a disaster, especially the fact that it coincided (and to some extent resulted from) a smallpox epidemic. From the perspective of other Native peoples in the region, however, the consequences were more complex. Many of them, like the Tlaxacans, were longstanding enemies of the Aztecs, who made war on them almost constantly. For them, the destruction of the Aztecs was at first a good thing, though in the long term the Spanish were at least as bad. In the end, whatever positive effects the conquests of Cortes may have had must be weighed against the fact that they were part of a larger process that resulted in the destruction of Native peoples throughout the Americas.