Considering the awful things the Aztec empire was doing it was probably a natural choice to ally with Cortes to overthrow it. But even if things are bad they can get worse:

In The Conquest of America, Tzvetan Todorov offers a compendium of some of the most chilling reports, mostly from Spanish priests and friars who, even when committed in principle to the belief that the extermination of the Indians was the judgment of God, could not disguise their horror at scenes of Spanish soldiers testing the blades of their weapons by eviscerating random passers-by, and tearing babies off their mother’s backs to be eaten by dogs. Such acts might perhaps be written off as what one would expect when a collection of heavily armed men—many of violent criminal background—are given absolute impunity; but the reports from the mines imply something far more systematic. When Fray Toribio de Motolinia wrote of the ten plagues that he believed God had visited on the inhabitants of Mexico, he listed smallpox, war, famine, labor exactions, taxes (which caused many to sell their children to moneylenders, others to be tortured to death in cruel prisons), and the thousands who died in the building of the capital city. Above all, he insisted, were the uncountable numbers who died in the mines:

"The eighth plague was the slaves whom the Spaniards made in order to put them to work in the mines. At first those who were already slaves of the Aztecs were taken; then those who had given evidence of insubordination; finally all those who could be caught. During the first years after the conquest, the slave traffic flourished, and slaves often changed master. They produced so many marks on their faces, in addition to the royal brand, that they had their faces covered with letters, for they bore the marks of all who had bought and sold them. The ninth plague was the service in the mines, to which the heavily laden Indians traveled sixty leagues or more to carry provisions … When their food gave out they died, either at the mines or on the road, for they had no money to buy food and there was no one to give it to them. Some reached home in such a state that they died soon after. The bodies of those Indians and of the slaves who died in the mines produced such a stench that it caused a pestilence, especially at the mines of Oaxaca. For half a league around these mines and along a great part of the road one could scarcely avoid walking over dead bodies or bones, and the flocks of birds and crows that came to fatten themselves upon the corpses were so numerous that they darkened the sun.”14