Major plot hole (****** BIG spoiler*******)
So right at the end the ghost of Abe kills Sherry (the nurse) in the ambulance by placing his hands on her chest and stopping her heart. He therefore prevents her being posessed and going on a killing spree.
Right through the film we have seen Abe (and before him presumably Caine) being haunted by ghosts trying to warn him that he is doing something bad by saving people and letting the devil into them.
So, if ghosts can kill the living as Abe does to Sherry, then why didn't any of these ghosts just kill a person who was saved before the three days are up? Thereby 'putting things right' again?
Or even why didn't one of them just kill Abe by stopping HIS heart, and therefore prevent him from 'saving' more people?
I prefer Imaginality to reality.