Spirits that wander the Earth move in two forms of existence. They're either moving in a robotic stasis or they've become aware, living a second life with a form of intelligence retained from their deceased human form.
There is a distinct difference become these ethereal bodies and demons. They are NOT the same. Depending on how they died, that energy..that spirit may become upset on the loss of their ability to interact with the living, to have an actual human body or very angry at how they died. Wandering this plane as a restless spirit is a punishment. There is no rest here.