28 Days Later

In Cambridge, three animal liberation activists break into a medical research laboratory. A scientist in the lab desperately warns them against releasing the captive chimpanzees, which are infected with a highly contagious rage-inducing virus. Ignoring his pleas, the activists release a chimp, which infects a female activist. She then attacks and infects everyone else present.
28 days later, in London, Jim (Cillian Murphy), a bicycle courier, awakens from a coma in St Thomas' Hospital. He finds the entire hospital deserted. He wanders the streets of London, finding it deserted as well, with signs of catastrophe everywhere. Jim enters a church and finds a priest, who turns out to be infected. Jim flees, attracting attention of more infected, but Selena (Naomie Harris) and Mark (Noah Huntley) rescue him. At one of their safehouses, they explain to Jim that while he was in a coma, a virus spread among the populace, resulting in societal collapse. They claim the virus has been reported in Paris and New York City as well, suggesting the infection has spread worldwide.
The next day, Selena and Mark accompany Jim to his parents' house in Deptford, where he discovers they committed suicide in bed together. That night, the three are attacked by more infected. Mark is bitten, and Selena viciously kills him. She curtly explains that the virus spreads through blood and saliva and overwhelms its victims in 10 to 20 seconds. She warns that should Jim become infected, she will kill him "in a heartbeat". The two see some blinking Christmas lights from Balfron Tower and head there. They discover two more survivors â cab driver Frank (Brendan Gleeson) and his daughter Hannah (Megan Burns) â who allow them to take shelter. The next day, Frank informs them that their supplies â particularly water â are dwindling. He plays them a pre-recorded radio broadcast from a military blockade near Manchester, claiming they have "the answer to infection" and promises to protect any survivors who reach them.
The group board Frank's cab and head to Manchester, bonding with one another throughout the trip. At the deserted blockade, Frank is infected when a drop of blood falls into his eye. He is killed by the arriving soldiers, who take the remaining survivors to a fortified mansion under the command of Major Henry West (Christopher Eccleston). West reveals to Jim that his "answer to infection" entails waiting for the infected to starve to death and luring female survivors into sexual slavery, to repopulate the world. The group attempts to flee, but Jim is captured and chained next to Sergeant Farrell (Stuart McQuarrie), a dissenting soldier. Farrell shares with Jim his speculation that the virus has not spread beyond Great Britain and that the country is simply being quarantined.
The next day, the girls are prepared for gang rape, while two soldiers lead Jim and Farrell to execution. When his executioners argue after killing Farrell, Jim escapes and spots a NATO aircraft flying overhead, proving Farrell's theory correct. Jim lures West and another soldier to the blockade, where Jim kills the latter and leaves West stranded for arriving infected. He runs back to the mansion and releases Mailer, an infected soldier West kept for observation. Mailer quickly spreads the infection among the soldiers in the mansion. In the confusion, a soldier drags Selena to the top floor to rape her, but Jim interrupts and brutally kills him. The two reunite with Hannah and run to Frank's cab. Jim is shot by West, who has been waiting inside the cab. Hannah reverses the cab towards Mailer, who grabs West through the rear window and kills him. The trio finally leave the mansion.
Another 28 days later, Jim is recovering at a remote cottage. Downstairs, he finds Selena sewing large swaths of fabric when Hannah appears. The three rush outside and unfurl a huge cloth banner, adding the final letter to the word "HELLO" laid out on the meadow. A lone Finnish jet flies over the landscape, and the infected are shown dying of starvation. The film ends with the jet flying over the three survivors and the pilot calling in a rescue helicopter.

Question: Who killed Mark?
Answer: Selena