Why were the Aztecs open to deception by the Spanish conquerors?

The Aztec civilization practiced human sacrifice and usually any surprise visitors were captured then sacrificed to the gods. However in 1500s when Cortes, the Spanish conquistador, arrived in the Aztec territory with a small party of men the Aztec’s did not realize the true purpose of their visit. The Aztecs thought that the Spanish were sent by Quetzalcoatl, their god. Quetzalcoatl was an important god for the Aztecs, they believed that the god was going to reappear when the world was coming to an end and save the Aztec people. At the time the Aztecs were under the impression that the world was coming to an end which is one of the reasons they were performing many human sacrifices. The sacrifices were believed to keep the god happy which might persuade him to put off ending the world for a time. When the Conquistadors entered the capital city of the Aztecs they had no idea that they were lucky just not to have been captured and sacrificed. The true reason for the Spanish being there was to see if there was gold and riches to be had and to convert the people to Christianity. The Spanish were welcomed by the Aztecs and even stayed as guests of the emperor. However, the longer the Spanish stayed in the capital city the more suspicious the Aztecs became. First of all the Spanish behaved nothing like the Aztec gods. They did not go to the sacrifices made in their honor. Eventually the Aztecs decided it was time for the Spanish to leave, they did not want to kill them however because they believed there was still a chance they actually were gods. The Spanish did leave but they returned they brought with them horses, dogs, weapons, and many men. They were also able to rally the support of the other tribes in the area who didn’t realize how horrible they would be treated by the Spanish. These other tribes just wanted to rid themselves of the Aztecs. Eventually the Aztecs were defeated but it was disease that caused it. The Aztecs did not have the same immunities that the Spanish had, when exposed to diseases like measles the Aztecs were decimated.