The Hills Have Eyes

A group of scientists are killed by a mutant named Pluto (Michael Bailey Smith). Later, Bob Carter (Ted Levine) and his wife, Ethel Carter (Kathleen Quinlan) are traveling from Cleveland, Ohio to San Diego, California for their silver anniversary. With them are their teenage children Bobby (Dan Byrd), and Brenda (Emilie de Ravin), eldest daughter Lynn (Vinessa Shaw), Lynn's husband Doug Bukowski (Aaron Stanford), their baby daughter Catherine (Maisie Camilleri Preziosi), and their German Shepherds, Beauty and Beast.
In the New Mexico desert, they meet a gas station attendant named Jeb (Tom Bower), who tells them of a short-cut through the hills. After taking the supposed short-cut, their tires are punctured by a hidden spike strip. Doug and Bob go look for help, while the rest of the family stays behind.
Bobby chases a panicked Beauty into the hills and finds her mutilated body; frightened, he runs and accidentally falls, knocking himself unconscious. A young female mutant named Ruby (Laura Ortiz) finds him and protects him from her brother Goggle (Ezra Buzzington). Bob goes back to the gas station for help, and upon investigation finds news clippings detailing various disappearances that have occurred in the area. He realizes the attendant purposefully led them to danger by suggesting they take an alternative route through the hills. When Bob exits he finds the hysterical Jeb, who commits suicide in front of him. In fear, Bob attempts to flee, but is attacked by the mutant leader, Papa Jupiter (Billy Drago), and dragged into the mining caves by Jupiter, Jupiter's eldest son Lizard (Robert Joy), and Pluto.
Bobby returns to the trailer, but doesn't mention Beauty's death not wanting to frighten the rest of the family. Later that night, the family is awakened by Bob's screams, and they all rush from the trailer, except for Brenda and the baby, who are left behind. Outside they find Bob being burned alive on a stake and frantically try to save him, but to no avail. Meanwhile, Pluto and Lizard use the distraction to ransack the trailer, where Lizard beats and rapes Brenda. When Lynn returns to the trailer she is greeted by Lizard and Pluto, and forced to let Lizard drink from her breasts while her baby is held at gunpoint. When Ethel returns she is shot by Lizard, and Lynn stabs Lizard in the leg, prompting Lizard to shoot Lynn in the head. Lizard and Pluto flee with the baby after Lizard attempts to shoot Brenda.
Doug and Bobby return to the trailer and discover Lynn and Ethel, who die shortly afterward. Goggle, who is watching the Carters with a pair of binoculars, is attacked by Beast, who tears his throat out, killing him. The next morning, Doug and Beast set off to rescue Catherine. Doug finds an abandoned nuclear testing village, and is knocked unconscious by Big Mama (Ivana Turchetto) during his search. Upon awakening, he escapes an ice box filled with human body parts and continues his search, only to encounter Big Brain (Desmond Askew). After Big Brain tells him the mutants' origins, Doug is attacked by Pluto, who severs two of his fingers with an axe. Pluto almost kills Doug, but Doug manages to gain the upper hand while Pluto is distracted, and kills him with his own axe.
Outside, Doug kills the mutant Cyst (Gregory Nicotero) and continues to search for Catherine. After ordering Lizard to kill Catherine, Big Brain is mauled to death by Beast. Lizard takes a cleaver and prepares to kill Catherine, but finds that Ruby has taken her. Doug sees Ruby running through the hills with Catherine and follows her. At the trailer, Brenda and Bobby prepare an explosive trap, which destroys the trailer, apparently killing Papa Jupiter.
Doug catches up with Ruby, but before she can return Catherine, Lizard attacks him. Lizard and Doug fight, and Lizard is presumed dead. When Doug turns his back Lizard aims a shotgun at him, but Ruby tackles Lizard off a cliff, killing them both. Bobby and Brenda find that Papa Jupiter managed to survive their trap, and Brenda finishes him off. They are then reunited with Doug, Catherine and Beast. As they celebrate their apparent victory, an unknown mutant watches them through binoculars from the hills.

Question: Who kills the mutant Cyst?
Answer: Doug