Shooter

Bob Lee Swagger reluctantly leaves a self-imposed exile from his isolated mountain home in the Wind River Range at the request of Colonel Isaac Johnson, who appeals to him to help track down an assassin who is planning to shoot the president. Johnson gives him a list of three cities where the President is scheduled to visit and Swagger assesses a site in Philadelphia as the most likely. This turns out to be a set-up; while Swagger is working with Johnson's agents to find the rumored assassin, the Ethiopian archbishop is instead shot while standing next to the president. Swagger is then shot twice by a city police officer, but manages to escape. The agents tell the police that Swagger is the shooter and stage a manhunt for the injured sniper. However, he meets a rookie FBI special agent, Nick Memphis, disarms him, and steals his car.
After his escape, Swagger takes refuge with Sarah Fenn, widow of his late spotter and close friend, killed years before in a mission in Africa. He later convinces her to help him contact Memphis with information on the conspiracy. Memphis is blamed for Swagger's escape and is informed that he will face disciplinary review, but argues that, given Swagger's training and experience, it is surprising that the president survived and the archbishop standing several feet away was killed. He independently learns that Swagger may have been framed for the assassination and is further made suspicious when he learns that the officer who shot Swagger was himself shot dead just hours later in a "mugging".
When the agents realize their secret is compromised, they kidnap Memphis and attempt to stage his suicide. Swagger tails them and kills the captors. The two then join forces and visit a firearms expert who provides information on the FBI's ballistics report and a short list of people capable of taking a shot from a distance of one mile or more. Armed with this, they plot to capture the ex-sniper, whom they think is the real assassin. Once they find him, he commits suicide after revealing that the archbishop was actually the real target and was murdered to prevent him revealing U.S. involvement in the massacre of an Eritrean village. The massacre was carried out on behalf of a consortium of American corporate oil interests headed by corrupt Senator Charles Meachum. Swagger records the exâsniper's confession of his involvement in the African massacre and then, with Memphis' assistance, escapes from an ambush by mercenaries.
Other rogue mercenaries kidnap Sarah to lure Swagger out of hiding. It is suggested that a restrained and bruised Sarah is raped by the mercenary, Jack Payne, who captured her. With his new evidence and cat and mouse strategy, Swagger and Memphis are able to rescue her when Colonel Johnson and Senator Meachum arrange a meeting to exchange their hostage for the evidence of their wrongdoing. Swagger shoots several men who are hiding to ambush him, also seriously wounding Payne, the mercenary. Sarah picks up a gun and finishes Payne off. The Senator is allowed to escape, while Swagger and Memphis surrender to the FBI.
Swagger is brought before U.S. Attorney General Russert and the FBI director in a closed-door meeting with Colonel Johnson, Memphis, and Sarah also present. Swagger quickly clears his name by loading a round into his rifle (which is there as evidence, since it was supposedly used in the killing), aims it at the Colonel, and pulls the trigger â which fails to fire. Swagger explains that every time he leaves his house, he removes the firing pins from all his guns, replacing them with slightly shorter ones, thus rendering them inoperable until he returns. Although Swagger is exonerated, Colonel Johnson cannot be charged with his crime, as the Eritrean massacre was outside American legal jurisdiction. The attorney general tells Swagger that justice does not always prevail in today's world. "It's not the Wild West where you can clean up the streets with a gun, even though sometimes that's exactly what's needed." Russert then orders Swagger released and exonerates him of all charges.
At the senator's home, Johnson and Senator Meachum celebrate and plan their next move. Swagger breaks in and kills both conspirators and their men, arranging for the house to blow up as if by accident. In a final scene, he drives away with Sarah.

Question: Who kills Payne?
Answer: Sarah.