The American Indian Tribes helped Cortes to conquer the Aztec Empire because they will benefit from its downfall.
The Aztec Empire practiced human sacrifice. They sacrificed 10,000 to 50,000 humans a year. These human sacrifices were either war captives or those kidnapped from neighboring communities. A lot of people fear this practice, thus, they assisted Cortes in conquering the Aztec Empire to put a stop to human sacrifice.