The Aztec empire fell because of various reasons. The ritual sacrifices contributed to the fall of the empire. Killing neighbors and other people isn’t such a good idea. The people who were sacrificed may have helped over power the Spanish.
Religion also contributed to the Aztec fall. It played a part because religion is basically the main reason for the sacrifices. They also believed at first that the Aztecs thought the Spanish were gods.

Disease contributed greatly in the fall of the empire. An African held as a slave had a smallpox, a very contagious disease. One of the men contracted the disease. When that one man died, an Aztec caught the disease. Then, smallpox spread among the population. Because this was a long time ago, people were not used to the idea of shots or medicines to heal the disease. In many cases, everyone in a house died and because they have no time to bury such huge number of people, they just demolished the house over the bodies. During the siege of Tenochtitlan in 1520, the population was dying. 25% of the population was said to have been lost because of the disease alone. Most importantly, the emperor died of smallpox, same with many leaders of the army.

(http://www.aztec-history.com/fall-of-the-aztec-empire.html)