The Woman

The movie opens with the feral Woman (Pollyanna McIntosh) circling what appears to be her child. A wolf, apparently tamed by the feral Woman, circles the infant as well but does it no harm. Although it is not referenced in the film, the Woman is the last remaining member of a cannibalistic tribe that has roamed the north-east coast for decades (as seen in the 2009 film Offspring).
Chris Cleek (Sean Bridgers) is a country lawyer at a local barbecue with his family. The oldest daughter Peggy (Lauren Ashley Carter) sits off to the side, upset. Their only son Brian Cleek (Zach Rand) watches as a couple of boys abuse and push a small girl into a corner, making no effort to save her. While out hunting, Chris happens upon the Woman, who is bathing. He returns with a net in an effort to capture her. He knocks her out and returns home with her, restrains her in a cellar, and directs his family to participate in "civilizing" her.
Over the following days, it is revealed that the family is dysfunctional. Brian enjoys causing pain to others, Peggy is withdrawn and afraid of her father and Chris gives the appearance of a smart, charming man. Chris' first attempt to approach the woman results in her biting off and eating the end of his ring finger. Chris and his wife Belle (Angela Bettis) protest and the youngest daughter Darlin (Shyla Molhusen) attempts to befriend the imprisoned woman. Chris' will prevails and he orchestrates a violent series of civilizing measures.
Chris bathes the Woman with boiling hot water and later decides to bathe her with a high pressure power washer. Belle watches in horror but Peggy rushes to the Woman's aid and turns off the washer. Peggy is forced into the house while Belle and Chris move the woman back into the cellar. Later, Chris rapes the Woman while Brian secretly observes. The next day, Brian also violates the woman and is caught by Peggy. Belle, unable to stand it anymore, announces her intention to leave Chris and take their two daughters but not their "rapist" son. Chris knocks Belle unconscious just before Peggy's teacher Ms. Raton (Carlee Baker) rings the doorbell. Ms. Raton tells Chris that she believes Peggy is pregnant. He becomes angry, knowing that she will somehow expose the family's secret and hits her. He and Brian tie her hands and drag her to the barn where he keeps German Shepherds. Peggy protests, but Chris subjects her to a vicious verbal assault.
In the barn, Chris and Brian lock Ms. Raton in the dog's cage. It is revealed that the cage also contains an eyeless girl named "Socket" (Alexa Marcigliano) who behaves like the two dogs. Socket and the dogs kill and eat Ms. Raton. Peggy releases the imprisoned woman from the cellar. The Woman attacks and kills Belle, gnawing off most of her face. She then kills Brian, severing his torso, and rips Chris's heart out and eats it. A terrified Peggy attempts to escape with her little sister. The Woman does not attack the girls. Instead, she takes Darlin and they walk away from the house together, along with Socket acting as a dog, apparently forming a new family. Peggy, resistant at first, follows the new family at a distance as the screen cuts to black.

Question: Who attacks and kills Belle, gnawing off most of her face?
Answer: The Woman