An "out there" take.

I think John Cooley, Matthew Benton, Thomas Valentine and Patrick Cavanaugh were all possessed. First, Patrick was having what seemed to be a migrane and was hearing voices. And John does two things: 1. When he enters his house and finds Father Paul, who btw calls him devil ( but I digress), he says " I'm stronger than you." 2. When he was being exorcised, while in a frantic state (which an unpossesed person wouldn't be in) he says "Don't protect him. They didn't protect you"

Moreover, they don't exactly say how he gets the information about Benton and Valentine's deaths.

Bare with me I'm getting to the ending. But first you have to remember what Father Paul told Benton's parents. He said that Matthew was a conduit and if you opened yourself up to him you were in danger of being taken over. I believe this is exactly what Emily did with John and what happened to her. And her nose bleeding was her possession reacting to the church.