The Aztecs sacrificed humans because they believed that without the sustenance of human life-blood, the sun would fail and the world would end. Their gods had sacrificed themselves to bring the world into existence, so humanity must continue the sacrifices to ensure the world remained sustained.
The Aztecs were, in many ways, an apocalyptic civilization, believing that light and darkness were in a constant struggle and that at any moment the sun could lose the fight. According to Encyclopedia Britannica, every 52 years, all lights were extinguished and people waited, breathless and terrified, for the Pleiades to reappear in the sky. When this happened, fires were re-lit and sacrifices resumed to provide extra energy to the gods fighting to maintain the world.