The Aztecs believed in an afterlife. After they died, the Aztecs believed they would be assigned a job to do that helped their gods. The job you were assigned or what you became in your afterlife did not depend upon how well you lived your life, but rather on how you died.
Warriors who died in battle were believed to turn into butterflies and hummingbirds. This helped the gods who created nature. This also helped to create a very strong army, with warriors who were not afraid of death. Warriors knew their afterlife would be spent in a most pleasurable way, unlike the people who died a natural death.
People who died of natural causes had the most unpleasant afterlife job. Their job was to serve the gloomy Lord of the Dead. The Aztecs believed it took about 4 years of dangerous travel, moving further and further down under the earth, to reach the Land of the Lord of the Dead. Once they arrived at their destination, these poor people lived their afterlife in darkness. It was not a punishment. It was just the way things were.
The afterlife job of women who died in childbirth was to help the sun god push the sun each day from its position overhead to a place where it disappeared for the night, so the sun could rest. This was not a bad way to spend your afterlife. The women only worked from noon till sundown each day. That was a much better afterlife job than those who died of natural causes.
People who died from lightning, drowning, and some diseases were believed to have been chosen to serve the rain and storm god. This was a good way to die in the Aztec religion. These people lived their afterlife surrounded by flowers.
Working for the gods in their afterlife was one more way the ancient Aztecs attempted to make their gods happy.