Many of the Aztec gods, such as Coatlicue and Huitzilopochtli, demanded human sacrifices. This was the driving force behind the "flower wars", or the wars between neighboring peoples not for conquest, but simply to get enough sacrifices to meet their needs.
There were two main ways of sacrificing a human. The first, more common method was to pin the victim down over a large rounded rock on top of a temple, and the high priest would take his obsidian or flint knife and quickly cut out the victim's heart, while it was still beating.
The second method of sacrifice was flaying, which directly paid tribute to Xipe Totec, the god of flaying. In this method, a victim was skinned, just like an animal. Sometimes, the priest even donned the victim's skin.
See drawings of sacrifices. Warning: the pictures are mildly graphic. Do not enter unless you think you can handle it.