The Ward

In rural Oregon, at the North Bend Psychiatric Hospital in 1966, a young patient named Tammy is attacked and killed by an unseen force during the night.
Kristen (Amber Heard), a troubled young woman, sets fire to an abandoned farmhouse and shortly thereafter is arrested. The local police take her to North Bend where she meets the other patients in the ward: artistic Iris (Lyndsy Fonseca), seductive Sarah (Danielle Panabaker), wild Emily (Mamie Gummer), and child-like Zoey (Laura-Leigh). Kristen is taken to a room, which the other girls inform her was previously occupied by their friend, Tammy. Kristen also meets her therapist, Dr. Stringer (Jared Harris). She is unable to recall anything about her past. Later, she awakens in the middle of the night and sees a horribly deformed figure staring at her. While she is with Iris and Emily, Kristen sees two people looking at her from Dr. Stringer's office, but the girls give no clue as to who they are. While taking a shower, Kristen is attacked by the deformed figure, but upon telling the nurse this, she is drugged and put through intense electroshock therapy. During a session with Dr. Stringer, Iris mentions Tammy but is then immediately halted by the doctor.
In a therapy session, Dr. Stringer uses hypnotherapy to unlock Iris' hidden memories. After the session, Iris is killed by transorbital lobotomy by the deformed figure. Kristen tries to discover what happened to her friend. In Iris's sketchbook, she finds a sketch of the figure that had attacked her, with the name Alice Hudson, a former patient at the hospital, written on top. That night, despite Sarah and Zoey's warning that it is nearly impossible to leave, Kristen and Emily attempt to find Iris and escape. However, the alarm is triggered. Kristen is thwarted by Alice and is rendered unconscious. She reawakens in her room, and it is revealed that Emily was caught.
On her way to an appointment with Dr. Stringer, Sarah flirts with a nurse but is rejected. She goes to the other girls and asks Kristen what happened to her plan. She also implies that they did not find Iris because they were "not looking into the right places." After bickering with Emily, she is electrocuted and killed by Alice. Kristen notices Sarah is missing. She learns that Zoey's doll formerly belonged to Alice, so she forces Zoey to tell her what is happening. She finds out that all of the girls had killed Alice, led by Tammy, because Alice constantly hurt them. Now Alice is after them for revenge. Emily attempts to commit suicide, thinking she is doomed. Kristen attempts to stop her, but Alice appears again and kills Emily by slitting her throat with a knife. Kristen plans a last attempt to escape by holding Zoey as a pretend hostage. Their attempt is thwarted by Roy. She is drugged and placed in a straitjacket. However, she manages to escape and takes Zoey with her to try and escape again. Zoey is captured by Alice and killed off-screen. After a lengthy chase around the hospital, Kristen seemingly manages to destroy Alice. Finding Dr. Stringer's office, she finds Alice Hudson's patient file, which details her treatments and each one of the girls' names, including Kristen herself.
Dr. Stringer, catching Kristen in his office, then reveals that Kristen is actually one of many personalities of the real Alice Hudson. It is also revealed that Alice was kidnapped as a young child and sexually abused by an unknown assailant eight years previous. Alice was left chained up for two months in the basement of the same farmhouse Kristen had burned down; in order to survive the trauma, her she developed multiple personality disorder, creating each one of the girls from the Ward as a different personality. Over time, Alice's own personality became so overwhelmed by that of the others that she became lost. Dr. Stringer attempted experimental techniques to bring back Alice's own personality back to reality, resulting in the manifestation of Alice that was destroying the individual personalities. He explains that her treatments were working until "Kristen" appeared, yet another invention of Alice's mind to protect itself from reliving the trauma back at the farmhouse. After this revelation, Alice is revealed to have survived its earlier encounter with Kristen and throws herself and "Kristen" out of the window, reawakening Alice.
Alice's parents, whom she had seen earlier in Dr. Stringer's office, come to take her home because she is finally fully treated. After gathering her belongings, Alice takes one last look around her room. Upon opening the medicine cabinet, Kristen suddenly comes out and attacks her. The camera cuts to black.

Question: Who kills Sarah?
Answer: Alice