The Aztecs were an empire meaning they conquered other people. They were a conquering empire just like Rome. That in itself is a really really difficult thing to do because if you conquer people you then have to rule over people and that's difficult to do.
The Aztecs were very brutal and they are most famous for human sacrifice. The numbers of people that they sacrificed is unimaginable and they had a very specific reason to sacrifice. They weren't cannibals necessarily but generally speaking, the Aztecs weren't cannibals but instead did human sacrifice for religious purposes.