They were religious events first. The Aztecs believed that their gods got their sustenance from human sacrifice; and one of the basic duties of Religion is caring for your gods. The most important of these sacrifices were carried out during the 18 monthly festivals of the Solar Year.
One of these, to give you an example, was the Tlacaxipehualiztli, the Festival of the Flaying of Men, celebrated at spring equinox before the rainy season, one of the most brutal and complex.
We know about it thanks to the notes of the Spanish monk Bernardino de Sahagun, who in the 16th century interviewed old Aztec men who were still alive in pre-spanish Mexico and recounted how this festival was held in the Aztec capital:
40 days (or maybe even a year) before the festival, a captive (from war) was designated to impersonate the god Xipe Totec (Our Flayed Lord), and he was celebrated in public as living image of the God until the Festival.
He was taught courtly manners, walking about the city playing a flute, smoking tobacco and being praised by the people and the Tlatoani (the leader).
He was even wed to four young maidens representing goddesses. There were similar representants for other important gods (Tonatiuh, Huitzilopochtli, Quetzalcoatl, Chililico and so forth).
These slaves-gods were to be sacrificed on the main pyramid by cutting out the heart. There were six sacrifice-priests who cut open the slaves breast with an Obsidian knife and then cut out the heart.
After that, the corpses were rolled down the pyramids stairs. The corpses were then flayed and their flesh given to important Aztecs. Moteuczuma would have gotten the best part, the femur. The flesh was then eaten.
Other captives would be clothed in the skin of the flayed corpses and adorned with the ornaments those killed earlier wore as “gods”.
They were paraded through the city by their captors, and finally, on the next day, fought in mock combat against Eagle- or Jaguar-wariors (they only had a mock sword with feathers instead of obsidian).
Once the captive was beaten down, he was sacrificed by a priest wearing the vestments of Xipe Totec.
His heart and blood from his chest was then presented to the sun. The captor would take that blood, and walk around the city to the statues of the gods, feeding them by painting their lips with blood.
The captives corpse was then brought to his captors house, flayed, and cut up, his flesh given away and eaten.
However, there was a special link between captor and captive, and the captor wouldn’t eat of the flesh of his captive.
Poor or sick people would walk through the streets, wearing the skins of the sacrificed, begging.
For twenty days, the priests, too, would wear the flayed skins, often adorned with gold and feathers, until the next festival (Tozoztli) approached.
The skins were then stored in special containers in a cave in the Xipe-Totec temple.
There were certainly festival-like elements, but the main events were very ritualized and everyone involved hat a part to play and knew what to do.
Even the captives were probably not struggling against their fate, but from what I’ve read, walked to the place of their sacrifice willingly, and played their part in the choreography.
The religious part was the most important. The gods needed to be fed.