It wasn’t the only system they had!!
The Aztec were a very brutal nation, not entirely unlike Sparta. ~20 years prior to Spanish contact the Aztec led the “Triple Alliance” of 3 major mesoamerican powers and conquered the surrounding countries to make an empire. The majority of the people in the empire did not like Aztec rule. The Spanish made an alliance with the original nations from the Triple Alliance, built a multinational coalition, and waged a war against the Aztec.
The Aztec and Spanish allies alike both suffered from the smallpox plague. The Spanish alliance emerged victorious and the Spanish, as the most powerful of the bunch on the winning side, took control of the newly formed state.
Moctezuma and Hernan Cortes were friends, they were good friends, when Moctezuma was betrayed and deposed, knowing he was going to be killed he asked Cortes to take care of his kids, and he did, he fought hard in Spain so that they could have titles.
Moctezuma was a Mexica, he was the King of one of the most brutal aztec tribes. History isn’t black and white and not easy.