There were also rival indigenous states (Tlaxcala, etc.) that were never conquered by the Aztecs but had been trapped in continuous low-intensity war with them for years.
Though it is worth saying the Aztecs weren't universally despised. Some regional dynasties were allies. Some of those were also unpopular.
And as most military based societies, Aztecs had also a big problem with management of ecology and natural resources, caused with their own development, big cities with a less than optimum management of fecal waters, and never ending wars.
Deforestation came with dry spells and agriculture suffered while they were trying to negotiate a solution with the stars. All the social structure was supported by a lot of drugs abuse, probably.
There was unrest and violence in the region long before Cortez arrived on the scene, when he did he was the external catalyst to ignite the powder keg. When reading accounts of the conquest, information gathering, political scheming and negotiations were often more "decisive" than the actual fighting.
Which is both likely true and morally against what we currently seem to believe.