Walled In

Sam Walczak (Mischa Barton), is a recent engineering graduate. At her graduation party, her father, an owner of a demolition company, gives her a gift: a job supervising the demolition of a building in the middle of nowhere. If she's successful, she will become his partner. Sam arrives at the Malestrazza Building, and is greeted by Mary (Deborah Kara Unger), the caretaker. Sam tells Mary she will be staying in one of the apartments in the building. Jimmy (Cameron Bright), the caretaker's teenage son, takes her bags to her apartment and explains the rules of the building. She is to stay off the eighth floor because it is Malestrazza's and the roof because it is too dangerous.
Sam researches the building and discovers that it was the scene of a horrible series of murders where sixteen people were entombed in the walls. Jimmy takes Sam to the eighth floor and tells her how his father was one of the victims, the crime being blamed on a local factory worker. The lights go out, and Sam cuts her leg after panicking in the darkness. Later, Jimmy washes her wound and begins to caress her thigh, but she tells him to stop. Sam begins having nightmares about being entombed in the walls.
Sam's boyfriend arrives, causing Jimmy to grow jealous. While exploring the eighth floor, Sam and her boyfriend discover a secret passage which would allow someone to secretly view any of the apartments. Sam realizes that Jimmy has been spying on her. Later, as Sam and her boyfriend make love, Jimmy watches from behind the walls. The next morning they find his dog, butchered. Sam and her boyfriend agree to leave.
Jimmy gives Sam a copy of the journal of Malestrazza, the building's architect. Sam realizes from reading the journal that there must be a large, open space in the center of the building. Jimmy goes to the roof to look for an opening to the hidden space. When he does not return, Sam and her boyfriend follow. They discover a massive shaft in the center of the building. They hear Jimmy calling from down in the shaft, saying that he is hurt. Sam has her boyfriend lower her into the shaft on a rope, but he is shot and killed, causing Sam to fall to the bottom. Jimmy then mails Sam's demolition report and sinks her car into a lake.
Sam awakens at the bottom of the shaft and discovers that a man is there with her. He is Malestrazza, the architect, who has been trapped there by Jimmy. He tells her that Jimmy is keeping her down there until she learns to love him. Sam also learns that Malestrazza was the mastermind behind the murders and is dumbfounded that people believe that a factory worker could concoct such a scheme of entombing people. He also tells her that if he does what Jimmy wants, Jimmy is nice, but can be very cruel and imaginative when he disobeys Jimmy's orders.
Jimmy is back at the hole and sends down a cassette tape. Malestrazza plays the tape, and Jimmy says that he wants to see Sam and Malestrazza dance. Sam reluctantly dances, but, when Jimmy tells them to kiss, she refuses. Malestrazza pulls her close and forces her to kiss him. After a few seconds, Jimmy tells them to stop and threatens Malestrazza not to touch Sam again. He sends down a basket with food, water, and a two-way radio. He tells Sam to keep the radio with her so that they can talk and be close.
Sam looks for a way to escape and realizes one wall leads to the garbage area. When Jimmy returns, she lies and says that she is hurt and needs medicine. When Jimmy brings her the medicine, she opens the capsules and starts creating a mix to try and blow out the wall to get into the garbage room. Sam's attempt fails, and Jimmy is upset that she tried to escape.
Before Jimmy could exact his punishment, his mother, Mary, catches him. Mary realizes that Sam is in the hole with Malestrazza. Mary tells Jimmy that place was for the architect not for anyone else. She realizes that Sam must stay in the hole with Malestrazza, or else they'll take her away and Jimmy will be an orphan.
Sam is stuck in the hole with Malestrazza, who she realizes does not want to escape. This is his tomb, and he picked her to kill him. She eventually does kill him after some taunting, and he thanks her after he falls into his tomb and it begins to fill with cement. Sam is by the wall hoping that Jimmy will come through and save her.
The demolition team arrives and her father asks where Sam is. Mary tells him that she left. Her father says that he thought she would want to see her first building being demolished. Mary is keeping Jimmy calm as they set up and begin to install the explosives. As the countdown begins, Jimmy starts saying "Sam," and then screams her name and runs toward the building. Sam's father tells them to stop the countdown. At the roof, Jimmy looks down, and her father asks what they have done. Jimmy then throws himself over, killing himself, and lands next to Sam. Sam is then lifted out of the hole and put in an ambulance.
Sam narrates that Malestrazza built 27 buildings and that they are all still standing. He considered the Malestrazza building his masterpiece. The building will not be demolished.

Question: Who is the mastermind behind the murders?
Answer: Malestrazza