"The Inferius is a corpse that has been reanimated by a Dark wizards spells. It is not alive, it is merely used like a puppet to do the wizard's bidding." - Severus Snape
Voldemort needed an army and his disregard for human life meant that he had no trouble sleeping when it came to waking the dead and using them as soldiers. Inferi are reanimated corpses, zombies if you will, of those who have passed away and Voldemort isn't giving them their right to rest in peace. In fact, the Inferi are probably the number one point of readers' con list of being a witch or wizard. Imagine sleeping in your bed at night with these guys crawling towards your door. No thank you.
