The Aztecs were an agricultural people, and as such, they considered the cycle of birth and death to be the essence of life itself. Since old crops must die before new ones can grow, these people recognized that death plays an important role in life’s cycle. Therefore death and mythology about the afterlife were a central part of the Aztec culture.

According to Aztec mythology, the spirit world controlled what happened in life, so they had to appease the gods in order to assure that things went well for the people on earth. This is why they sacrificed humans. Just as they made offerings of corn or other crops so the gods would let new crops grow, they sacrificed human beings so that those gods would allow the human race to thrive. Those who were killed in sacrificial ceremonies were not seen as victims, but rather as privileged beings, chosen to deliver messages to the gods. This got their souls into the most desirable place in the afterlife. 

To the Aztecs, the method of death was the determining factor in what kind of afterlife a soul would find. They believed in thirteen levels of heaven with different features in each of the various paradises. At the time of death, a soul returned to the world of spirits for a period of time.

The souls of those who were sacrificed went directly to the eastern paradise, which was the house of the sun. The only others that obtained this high honor were the souls of those who died in battle.  It was believed that these souls stayed in paradise for four years before returning to the earth in the form of hummingbirds or other exotic birds.

The spirits of women who died in childbirth went to the western paradise, which was thought to be the house of corn. Those who drowned, died of lightening, or of any diseases thought to be caused by water went to the southern paradise.  

The souls of those who died of any other cause had to go through nine levels of the underworld before they were allowed to rest in the paradise of the north.  According to Aztec mythology, it took four years to do this, during which time the soul had to endure many trials before finally reaching the level of rest.

Each soul had to cross a deep river, pass between two large, conjoined mountains, climb another mountain made of jet-black volcanic glass, endure a wind that was icy and cutting, go through a place with many flags waving, be pierced and punctured by arrows, walk among wild beasts who ate human hearts, and stumble along a narrow stone path. 

At the time of burial, people were given magical powers, and they were often buried with items that might help them make their way through these ordeals so that they could find their final resting place in Mictlan, the lowest level of the underworld. 

They did not rest forever. According to Aztec mythology, souls eventually left the spirit world to re-enter that of the physical world. Thus, the cycle of birth and death continued.

Sources:

http://www.river-styx.net/aztec-myth-mictlan.htm

http://www.answers.com/topic/tlalocan