Slaughter Hotel

A hooded, axe-wielding killer lurks around a large rural villa which has been converted into an asylum. It begins when a woman, named Ruth, is committed to the clinic by her husband. She attempts to escape by assaulting an orderly as well as attempt suicide, but is restrained. One of the residents, named Cheryl, is visited by her husband, Mr. Hume, who had committed her because of a suicide attempt due to her stressful job at working at his company. Mr. Hume talks with the clinic director Dr. Francis Clay and his associate, Dr. Austin, about the possibility of Cheryl being cured. Dr. Clay tells Mr. Hume that Cheryl's suicidal urges may relapse once she is released, but Hume thinks that his wife only needs some more rest at the clinic.
Meanwhile, Helen is a nurse who is tending to resident Mara, who tells Nurse Helen that she seems to be improving with her treatment. Another patient is Anne who is a diagnosed nymphomaniac. Anne attempts to follow the gardener to seduce him, but she is called back to her room by Dr. Austin who counsels her about her "impulsive" and "excessive" sexual desires. Anne talks to Peter, an orderly, that she is getting better. Anne says that no one can calm her "passions" like Peter, but Peter is evidently not as sexually interested in the way that Anne seems to remember.
Later that evening, as the attendants and patents sit in a room to mingle and play cards and board games, Anne sneaks out the front door and runs to the greenhouse. The hooded and cloaked person is outside, and after a nurse walks by (seeing and ignoring the person), she is beheaded with a scythe.
Anne sees the gardener, takes off all her clothes, approaches him and seduces him into having sex with her in the greenhouse. Meanwhile, Helen goes to Mara's room and tells her that she can join the others if she wants and says that she will check on her later. Dr. Austin is told that Anne is missing, and the attendants go to find her. After having sex with Anne, the gardener tells her that she must leave for he will suffer the consequences of their tryst. Anne does not want to leave, so the gardener smacks her. Anne hits him back, calmly puts her clothes back on and leaves. She walks over and kisses the male attendants that find her until Austin calls her to stop. Meanwhile, Cheryl asks Dr. Clay if she will be like she was before, and the doctor tells her that she has been cured.
The killer goes back inside the clinic, gets a knife, and unlocks Ruth's door as she sleeps. The killer takes off his hood and Ruth awakens and she goes for the killer with the knife, but she gets knocked aside and the killer then chokes her and stabs Ruth in the chest.
Dr. Austin looks around the hallway with a flashlight and finds Nurse Helen, who says that she heard a noise. A chauffeur enters the building and drinks all of the drinks leftover from the get-together. After looking around, the killer shows up and murders the chauffeur by pushing him into an iron-maiden-esque device, and his blood pours out. The killer next walks around with a sword and angrily hits on the bed in Cheryl's empty room. Cheryl meets with Dr. Clay in the hallway and they leave together. Dr. Clay and Cheryl talk of their potential relationship, and he leaves to "do the rounds".
While Nurse Helen enters Mara room and clearly attempts to seduce her while Mara is taking a bath, the killer walks into Anne's room while she sleeps, shuts her window, and takes off his hood. Anne's eyes open, she sees the killer standing over her bed and asks him to lie down next to her. The killer chooses instead to axe her to a bloody death.
Back in Mara's room, she dances a bit for Helen to a song on the radio. As Mara looks out of her window, the killer fires a crossbow at hits Marta in the neck with an arrow, killing her instantly. Helen screams and a crowd gathers outside her door. Austin and Clay see the dead woman and Austin tells an attendant not to let anyone in the room. The two doctors and Cheryl look around the building when they find blood on the antique weapons and discover the body of the chauffeur. Clay points out that the other sword from the display is missing and Austin finally calls the police, stating to the police commissioner that the killer is still inside the clinic.
The police and medical attendants remove the bodies of Anne, Ruth, Mara and the other nurse from the area, but they are angry that Dr. Austin moved some weapons and tried to keep the killings a secret, thus making him an accessory after-the-fact. The police inspector suggests using Cheryl as bait, and Dr. Clay tries to get her to rethink the idea after she agrees to it.
While all of the other clinic patents are moved into a single room for their own protection, Cheryl awaits in her room. The killer approaches with a rope to strangle where he removes his hood... finally revealing himself to be her husband Hume. The police detectives show up before he can finish strangling Cheryl and chase Hume around the building. Clay and Austin theorize that Cheryl's husband wanted to kill his wife for some time and he created the idea of a maniac spree killer so that no one would suspect him of Cheryl's murder. Hume knocks out two of the cops chasing him and upon running into a room to hide, discovers all of the women patents in it. Hume goes on a brutal killing spree, killing every single woman in the room before the police run into the room and finally shoot Hume dead.

Question: What kills Mara?
Answer: Arrow