context: Tensions are running high in the United States. The recently deceased are returning to life as shambling, flesh-eating ghouls, and political dissidents are threatening social stability. In the city of Philadelphia, Fran (Gaylen Ross) an employee of WGON-TV and Radio learns that her program director Givens is broadcasting out-of-date information about rescue stations that have been overrun by the living dead. Fran's boyfriend Stephen (David Emge), who pilots the station's traffic helicopter, arrives at the station and tells her that they must escape in order to survive. He plans on stealing the station's helicopter and tells Fran to meet him on the roof at 9pm.Meanwhile, in another part of Philadelphia, the city's SWAT team is preparing to launch an assault on a low-income tenement building filled with refugees who refuse to leave their homes for emergency shelters; they also refuse to give up their dead, both deceased and re-animated, harboring them in the basement of the tenement. After gun battle with armed bandits on the roof, the police and National Guard soldiers break into the building using tear gas. Roger (Scott H. Reiniger) one of the SWAT troopers, becomes disgusted by the bloodthirsty behavior of some of their fellow SWAT members when one of them goes crazy and begins shooting at both zombies and innocent bystanders. A tall figure in a gas mask shoots the rogue officer dead. Roger retreats to the basement to escape the confusion, but the tall figure follows him. He removes his mask and reveals himself to be an imposing man named Peter (Ken Foree). They have talk for a few minutes and agree that the zombie plague will only get worse. Roger tells Peter about his friend who has a helicopter; he plans to leave with Stephen and Fran, and suggests Peter join their escape. A priest emerges from a room in the cellar; the man is missing a leg and walks with a crutch. He tells them that he has administered the last rites to the dead people in the next room, and that the soldiers may now "do what they will" with them. Peter and Roger kill all the zombies in the cellar, the gravity of the situation nearly overcoming them.Meanwhile, Stephen and Fran arrive in their helicopter at an abandoned police station pier on the Delaware River to refuel and wait for Roger to show up. They are surprised when four rogue policemen arrive and threaten them for looting the place. Roger and Peter arrive and the rogue cops back down allowing the four to go on their way. Stephen and Fran are surprised that Roger has brought someone with him, but they agree to take Peter with them. The four of them fly all night.In the morning, Stephen lands the helicopter at a small, deserted rural airport depot to refuel. While exploring the area, zombies attack them from out of nowhere. Stephen tries to prove his manliness before Fran -- who is carrying his child -- but he is both clumsy and a poor shot. Roger has to kill the zombies Stephen weakly shoots at, and Stephen nearly kills Peter when aiming for a zombie.Later, while in the air again, they see a deserted shopping mall. They land on its roof and break in through a skylight to rest and to collect supplies. The place is full of zombies milling about inside, drawn to a place that had meant so much to them when they were alive. Despite the growing number of undead outside, the four refugees realize that they could survive there for a long time. It even has a gun shop so they will be able to find weapons to defend themselves. Using the mall's floor plans and master keys, Peter, Roger and Stephen race around the mall, collecting the supplies they will need to make a home. They take these supplies to an upstairs area where they and Fran build a home in an annex of former offices and storage rooms. Fran is resistant to the idea; she thinks it is a bad idea to leave what is left of society, but perhaps because she is pregnant and wants to do what's best for her child, she stays.They realize they need to rid the mall of zombies. A decision is made to block the entrances to the mall with semi-trucks from a nearby warehouse. While Stephen watches from his circling helicopter, Peter and Roger hotwire the trucks and drive them to the mall. Peter is disturbed that Roger seems to be losing his grip on reality; he becomes arrogant and takes unnecessary risks in order to demonstrate his mastery of the situation. Unfortunately, he is bitten by zombies while trying to get into one of the trucks. They all realize that Roger's wound will prove fatal, as nobody can survive a bite for more than a few days. Fran is conflicted; she wants to take Roger to a medical facility, but the others won't hear of it. They are convinced Roger is doomed but decide to let the "disease" run it's course.Over the next several days, they begin to make the mall their home. They care for the ailing Roger as best they can, but his condition rapidly deteriorates until it is clear he is on the verge of death. Peter builds a fake wall in front of the only hallway that leads to their home, disguising it from any potential looters that may come through the area. They monitor a television to see if the zombie crisis ever ends but the news is dismal. Roger makes Peter promise to kill him if he comes back as a zombie, because he tells Peter he's going to try not to return. Roger does return, however, and Peter is forced to shoot his animated corpse in the head. They bury Roger inside of a large planter that houses decorative palm trees inside the mall.One night Peter prepares a romantic dinner for Fran and Stephen. When Peter leaves them alone, Stephen proposes marriage to Fran. Fran declines, saying that they can't make that kind of commitment given what has become of the world. "It wouldn't be real," she explains.Eventually, all television transmissions cease. The remaining three bicker as their sense of isolation gets stronger. Fran demands to learn how to fly the helicopter in case anything were to happen to Stephen. But when he teaches her how to fly it, they are spotted by a gang of motorcycle-riding survivalists.That evening, the survivalists/looters approach, coveting the supplies inside the mall. They try to contact Peter, Fran and Stephen by radio, but Peter is smart enough not to respond. He knows that they will be killed if the survivalists find them. His plan is to let the looters break in and take what they want, then leave, betting that they won't find the camouflaged entrance to their home. The looters break into the mall, and zombies flood in with them. Stephen, believing that the mall belongs to him and his friends, sneaks into the mall and tries to shoot the bikers, who return fire. With no choice but to help his friend, Peter creeps through air shafts, picking off isolated bikers and zombies. Unable to cope with both Peter and the zombies, the bikers decide to leave. Before they do, two of them enter the elevator where Stephen is hiding on top, and begin blindly shooting, severely wounding him. He is later attacked and bitten by zombies, and dies in the elevator. In the mall, the zombies corner and eviscerate several of the remaining looters.Peter returns to the hidden fortress and waits with Fran; they are unsure if Stephen has survived, but they know he will return either way. Now a zombie, Stephen remembers how to get through the hidden wall to the storeroom. He leads the horde of zombies up the stairwell. When Stephen appears, Peter kills him. He tells Fran to leave, but he refuses to go with her. He helps Fran onto the roof, where she prepares the helicopter. Peter stays in the storeroom, waiting for the zombies to flock in and planning to shoot himself before they can grab him. But at the last minute he decides he wants to live and fights his way through the zombies to the helicopter. Fran delayed taking off until the last possible moment, and Peter is able to hop into the helicopter. The two fly off to an uncertain future, with little fuel left.
question: Why does Peter kill Stephen?
answer: He had been turned into a Zombie (Dawn of the Dead 1978)