George A. Romero's Survival of the Dead

The prologue follows the actions of National Guard Sergeant "Nicotine" Crockett, who along with Kenny, Francisco and Tomboy, desert their posts and rob the protagonists of the previous film. Meanwhile, off the coast of Delaware lies Plum Island, home to two feuding Irish families: the O'Flynns and the Muldoons. The former family, led by Patrick O'Flynn, round up a posse to kill the undead on the island. O'Flynn learns that the Muldoons are keeping their undead loved ones safe until a cure is found. Tensions come to a head when O'Flynn and his posse arrive at a Muldoon house to dispatch their undead children, only to engage in a brief gunfight that leaves a woman dead as well. Unable to put the children down himself, Patrick surrenders his weapons when the Muldoon posse arrives. Seamus Muldoon contemplates killing Patrick until Patrick's daughter Janet suggests he be exiled from the island instead.
Boy joins the National Guardsmen and through him they learn of Plum Island. They watch a video made by Patrick O'Flynn and follow the instructions in it that leads them to a nearby dock. At the dock, O'Flynn and his men attempt to rob the Guardsmen, which results in a shootout. Francisco steals a ferry boat and bites off the finger of an attacking zombie in the process. All of the O'Flynns but Patrick are killed by zombies, and he boards the ferry. During the trip to the island, Patrick says that he sent other people to Plum Island to anger the Muldoons.
When the group reach the island they discover that the Muldoons have chained up their zombies in imitations of their previous lives. They also see that the people sent to the island by Patrick have been killed. Patrick sees his daughter Janet ride by on a horse, apparently dead and turned into a zombie. Patrick attempts to gather allies when two Muldoons attack them, shooting Crockett and Kenny. The latter dies from his wounds, and is shot in the head by Patrick to prevent reanimation. Francisco realizes that he infected himself when bit off the zombie's finger and asks Tomboy to shoot him to keep him from turning. Tomboy shoots him and is then captured by Muldoon.
Patrick finds out that the daughter he saw earlier was actually Janet's twin sister Jane. Janet, still alive, joins Patrick and the Guardsmen in their attack on the Muldoons. A standoff occurs at the bridge that separates the two families' land, and the O'Flynn group is captured; Boy and Janet escape. Muldoon reveals his attempts to persuade the zombies to eat something other than human flesh and uses Jane as a test case. He tries to persuade the dead woman to bite her horse, but instead she attacks and bites her sister Janet. A melee ensues and captured zombies are released, consuming people from both sides. Muldoon and O'Flynn call a truce that is almost immediately broken when Muldoon shoots O'Flynn, who pulls a hidden gun and kills Muldoon.
Crockett and his group attempt to leave the island. Janet witnesses her sister bite the horse and rushes to tell Crockett's group the news. Patrick shoots his daughter to prevent her turning before he succumbs to his own wounds. Crockett, Boy, and Tomboy board the ferry and escape the island while the zombies are eating the horse. Crockett muses about the purpose of war as the reanimated O'Flynn and Muldoon stagger toward each other and attempt to shoot and kill the other again.

Question: Who killed all of the O'Flynns but Patrick?
Answer: zombies