The Last Exorcism

Reverend Cotton Marcus (Patrick Fabian) lives in Baton Rouge, Louisiana with his wife and son. Marcus, who lost his faith after the birth of his ill son, is accustomed to performing fake exorcisms on "possessed" individuals. He chooses an exorcism request sent by farmer Louis Sweetzer (Louis Herthum), who claims his livestock is being slaughtered by his daughter Nell (Ashley Bell) who is possessed.
After listening to the details of the case, Marcus claims Nell is possessed by a powerful demon named Abalam. Prior to the exorcism, Marcus bamboozles the family into believing he is driving out a demon. After the ritual, Marcus and his film crew leave, believing they have cured her of a mental state that was misdiagnosed as a possession. That night, Nell appears in Marcus' motel room, apparently unwell. Marcus takes Nell to the hospital for tests, which conclude that Nell is in perfect physical condition. Marcus goes to see Louis' former pastor, Joseph Manley. Manley informs Marcus that he has not seen Nell for two years. In the morning, Louis takes Nell home, but chains her to her bed after she slices her brother Caleb's (Caleb Landry Jones) face with a knife. That night, the crew find a drawing of a dead cat.
Nell steals their camera and goes into her father's barn, where she brutally smashes a cat to death while filming it with a camera. She returns to the house and approaches Marcus with the camera, and just as she's about to make contact, the crew stop her. The crew then discover two more of her paintings. The first depicts Marcus standing before a large flame, holding up a crucifix. Also depicted is Iris cut into pieces, and Daniel decapitated.
When everyone learns Nell is pregnant, Marcus accuses Louis of incest, which he denies, insisting that Nell is a virgin and has been defiled by the demon. The crew has a confrontation with Nell, who slashes Marcus's hand with a pair of scissors and then flees. The crew decide to leave, and when they see Nell sitting on the porch, Marcus approaches her and she tackles him. Louis is about to kill Nell with his shotgun, while Nell is begging him to. To keep Louis from killing her, Marcus offers to attempt a second exorcism.
Marcus confronts the entity, Abalam, who agrees to release Nell only if Marcus can remain silent for ten seconds. Abalam begins breaking Nell's fingers as it counts aloud. After breaking three of her fingers, Marcus yells for Abalam to stop. Abalam then asks Marcus if he wants a "blowing job." Marcus realizes that a demon would know the actual name of the sex act and concludes that Nell is not a demon, but a very disturbed girl. Nell anguishes over losing her virginity to a boy named Logan, resulting in her mental breakdown rather than possession. Marcus arranges for Pastor Manley to come to the house, and leaves. On their way home, Marcus and the crew meet Logan, who explains that the only contact he had with Nell was a brief conversation 6 months ago at a party held at Pastor Manley's home. Logan admits that he is gay and therefore did not have sex with her. Marcus realizes that Pastor Manley was lying about having not seen Nell for two years. He returns to the Sweetzer farmhouse, where they find numerous occult and countercultural symbols on the walls, and Nell and Louis missing.
Marcus and the crew wander into the woods, where they see a large fire and a congregation of hooded cultists, led by Pastor Manley. Louis is tied up, gagged, and blindfolded while hooded figures pray around an altar, atop which Nell is bound. She gives birth to an inhuman child. Manley throws the child into the fire, which causes the fire to grow rapidly as demonic roars emanate from within. At that moment, Marcus' faith is restored, and he grabs his cross and rushes towards the fire to combat the evil. Iris and Daniel are discovered; Iris is tackled by a member of the congregation and killed with an axe. Daniel continues to run, until he stops to take a deep breath. He then looks around to see if he was followed, before turning back to find Caleb, who is wearing clean clothes and a bandage covering his wound. Caleb then decapitates Daniel and the camera falls to the ground, before it cuts out.

Question: Who slashes Marcus's hand?
Answer: Nell.