Were the Aztecs cannibals?

Yes. But it was a political necessity used to intimidate enemies as well as weaker neighbors. For keeping an empire in tact, the Aztecs fought back the Spaniards as well as disloyal friends and other enemies in the way it was known - captives who invaded the land would be sacrificed in a practical ritual to please the gods for saving the nation from intruders. On live person they cut a chest open, reap a beating heart out and offer it to the Sun. Then the body was dismembered, cooked and eaten. Well, those days long gone, and history was written by the victors (Spaniards again who claimed that Aztecs gave in without resistance. But the fact is no more in limbo. It's proven and history is in balance. Indigenous people did not give their land to outsiders without battle : )