The ultimate goal of any demonic entity is the possession of a human body (and its attendant brain) and the enthrallment (enslavement) of its owner’s soul. Exactly what happens to the human soul if a demon that has actually managed to capture it and eventually kills the body is unclear. It is widely assumed that the soul is either enslaved or possibly destroyed. Perhaps in addition to stealing bits of psychic energy from all the evil people in the world, Hell gets part of its energy by the capture of human souls. Remember that Hell is cold and dark. As “beings of light”, all humans are a great source of spiritual energy. Maybe devils and demons need to capture human souls for the spiritual energy they contain.
On the other hand, we know that a human soul is immortal. If it can’t be destroyed, the demon will at minimum attempt to derail the soul’s spiritual journey, perhaps stealing as much energy as it can during the soul’s entrapment. Each human soul is a tiny piece of God’s consciousness, and it seems reasonable to assume that God would not allow the total destruction of one of His own children. Perhaps these souls will survive, regain the use of their free will, and eventually find their way back to God.
Infestation of a house or possession of a human body has one major advantage for a demon. It allows the demon to temporarily escape its miserable conditions in Hell. Hell is dark, cold, noisy and incredibly smelly. The residents of Hell all know that when they rebelled against God, they surrendered their right to live in heaven. They hate God for their banishment, and they hate human souls because they are God’s children.
Demons not only torment humans whenever they can, but they torment each other when not tormenting humans. They live in an environment that is dark, cold and depressing. By possessing a human body, they can attain at least a temporary respite from the usual torments of Hell. As a secondary benefit, they can also experience the demonic joys of rape, murder, mayhem and myriad other crimes in the material world.
Demons have only one ultimate purpose: That is to harm God in any way they can. Their most effective weapon against God is to harm or to destroy one of His children. By hurting, or imprisoning a human soul, they are hurting God in the same way that willfully hurting or killing a child hurts its mother. That is the penultimate aim of every demon and every devil. To desecrate man and derail his spiritual journey, even temporarily, is to profane God’s work.