While we do have a The Last Exorcism review where you can leave comments, we've set up this as a place where you can discuss the Last Exorcism ending and movie spoilers without concerns about ruining the movie for folks who haven't seen it yet.
If you're posting comments here, assume that anyone in the conversation has seen the movie - if you haven't seen the movie, I would recommend you don't read the comments here until you have. :)
So... the Last Exorcism ending that most everyone who reviews the movie is talking about - how DOES it end?
As mentioned in my review, what the filmmakers attempted to do was to NOT give the audience a definitive answer throughout the movie to the question of whether Nell was actually possessed or going through some severe psychological trauma. That "not knowing" kept the movie interesting - and watching the pseudo-exorcist preacher (named Cotton) start out thinking this was just a case of a mental problem but start to doubt himself as the film went on takes the audience along the same thought process.
At one point during the film we learn that Nell's father had a falling out with the local town pastor, and that's why he stopped taking his family to church (he's also home schooling 15 year old Nell and her teenage brother). Cotton talks to the preacher, who says that it got pretty ugly between him and Nell's father and that while he knows an excellent pyschologist, he doesn't believe the father will listen to him.
Cotton talks them both into speaking with each other, and goes back to the house in the woods to perform (supposedly for real this team) another exorcism with the local preacher and his secretary present. Cotton leaves, thinking everything is finally settled, but it's at this point he and the camera crew discover the boy they thought impregnated Nell is gay.
So at this point, within the last 10 minutes of the film, we're fairly convinced that her problem stems from a combination of her mother's recent death and the fact that she was recently raped but has suppressed any memory of it. At first Cotton believes Nell's father raped her, and then the suspicion falls on the young man in town, but he's not the guy. Cotton and crew turn around and head back to the house.
When they arrive they find satanic symbols painted all over the walls of the house and no one home - they head out in the dark towards a distant light and they can hear screaming in the distance. When they get there, they come upon what looks like a satanic ritual with a ton of people from town, Nell on her back apparently giving birth to a creature that is not altogether human (she didn't look the least bit pregnant). It's revealed that the leader of this occult group and the man who impregnated Nell is, you guessed it, the local town pastor.
The newborn creature is thrown screaming into the fire, which blazes into a huge fireball when this happens - Cotton, seeing all this, seems to try with all his might to regain his faith in God, raises a cross and heads towards the melee. The cameraman and the woman doing the interviewing for the documentary run off terrified into the woods - she is struck down and presumably killed and the cameraman keeps running. Eventually he is cut off by Nell's brother who decapitates the cameraman, the camera falls to the ground, and the screen goes black.