It wasn't just the captive slaves and human sacrifice either (though that was a big deal), the Aztecs imposed crushing economic tribute burdens on subjugated city-states, and when payments fell short, they'd go in and raid. Tribute wasn't just gold and tradeable goods, it was also food, and the Aztecs didn't care if they drove a place into famine.

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.