Dawn of the Dead

The United States is devastated by a mysterious phenomenon which reanimates recently deceased human beings as flesh-eating zombies. Despite the best efforts by the U.S. government and local authorities to control the situation, society is beginning to collapse. Some rural communities and the military have been effective in fighting the zombies in open country, but cities are helpless and largely overrun. Confusion reigns at the WGON television studio in Philadelphia by the phenomenon's third week, where staff members Stephen Andrews and Francine Parker are planning to steal the station's traffic helicopter to escape the zombies. Meanwhile, police SWAT officer Roger DiMarco and his team raid a housing project where the residents are defying the martial law of delivering their dead to National Guardsmen. Some residents fight back with handguns and rifles, and are killed by the combined might of the overzealous SWAT team and their own reanimated dead. During the raid, Roger meets Peter Washington, part of another SWAT team, and they partner up together. Roger tells Peter that his friend Stephen intends to take his network's helicopter, and suggests that Peter come with them. The matter is decided when they are informed of a group of zombies sheltered in the basement, which they execute with grim determination.
That night, Roger and Peter escape Philadelphia with Francine and Stephen in the helicopter. Following some close calls while stopping for fuel, the group comes across a shopping mall, which becomes their sanctuary. Francine reveals that she is pregnant. Peter offers to abort the child, but this is rejected. Stephen teaches Francine how to operate the helicopter in case of emergency. To make the mall safe for habitation, they block the entrances with trucks to keep the undead masses outside from building up enough cumulative force to break through; they also craft a wooden "false wall" to hide the access to their living space. During the blockade operation, Roger becomes reckless and is bitten and infected. After clearing the mall of zombies, the four enjoy a hedonistic lifestyle with all the goods in the shopping mall available to them. Roger eventually succumbs to his infection, soon reanimates and is shot in the head by Peter. All emergency broadcast transmissions eventually cease, suggesting that civilization as they know it has completely collapsed.
A gang of nomadic motorcyclists, having seen the helicopter during one of Francine's flying lessons, break into and start looting the mall, destroying the barriers and allowing hundreds of zombies inside. While Peter suggests they let the bikers take what they want and move on, Stephen foolishly starts a gun battle with the bikers and is shot in the arm. He tries to escape through an elevator shaft, but is cornered by the undead and bitten several times. As some of the bikers, shot by Peter, are consumed by the zombies, the rest retreat with their stolen goods. A reanimated Stephen, apparently knowing enough to remember the false wall, breaks through it and leads the undead to Francine and Peter. As Stephen enters their hideout, Peter kills him while Francine escapes to the roof. Peter then locks himself in a room and contemplates suicide. When zombies burst into the room, he has a change of heart and fights his way up to the roof, where he joins Francine. The two then fly away in the partially fueled helicopter to an uncertain future.

Question: Why does Peter kill Stephen?
Answer: zombie