Twin Peaks: Fire Walk With Me

This article's plot summary may be too long or excessively detailed. Please help improve it by removing unnecessary details and making it more concise. (April 2016) (Learn how and when to remove this template message)
FBI Regional Bureau Chief Gordon Cole (David Lynch) sends agents Chester Desmond (Chris Isaak) and Sam Stanley (Kiefer Sutherland) to investigate the murder of drifter and teenage prostitute Teresa Banks in the town of Deer Meadow, Oregon.
Desmond and Stanley view Teresa's body at the local morgue following a tense exchange with the town's sheriff. They notice that a ring is missing from her finger and a small piece of paper printed with the letter "T" has been inserted under one of her fingernails. The agents question residents about Teresa but come up with few clues so Stanley leaves Deer Meadow while Desmond remains behind. Desmond discovers Teresa's missing ring under a trailer belonging to the sheriff's deputy. As he reaches out to it, the screen fades to black.
The following day at FBI headquarters in Philadelphia, Cole and Agent Dale Cooper (Kyle MacLachlan) experience a brief vision of their long-lost colleague Agent Phillip Jeffries (David Bowie). In a surreal montage, he tells them about a meeting he witnessed involving the Man from Another Place, Killer Bob, Mrs. Chalfont (Frances Bay) and her grandson, all mysterious spirits who appeared as characters in the original television series. Desmond is reported missing and Agent Cooper is sent to Deer Meadow to investigate. The clues to Teresa's murder lead to a dead end but Cooper is certain her killer will strike again.
One year later in Twin Peaks, high school homecoming queen Laura Palmer (Sheryl Lee) and her best friend Donna Hayward (Moira Kelly) attend school. Laura is addicted to cocaine and is cheating on her boyfriend, the arrogant and ill-tempered jock, Bobby Briggs (Dana Ashbrook), with the biker James Hurley (James Marshall).
Laura realizes pages are missing from her secret diary and tells her friend, the agoraphobic recluse Harold Smith (Lenny Von Dohlen) that Bob took them. Laura insists that Bob is real and will kill her if she does not allow him to possess her. Laura's face then changes color as she utters the words, "Fire walk with me." She quickly returns to normal and, visibly shaken, gives Harold her diary to keep. She leaves saying she does not know when, or if, she will return.
During her Meals on Wheels rounds, Laura sees Mrs. Chalfont and her grandson. They give Laura a painting and warn her that the "man behind the mask" is in her bedroom. Laura runs home, where she sees Bob. She rushes outside in terror and hides in the undergrowth, then sees her father, Leland (Ray Wise), emerge from the house. Laura begins to suspect her father is Bob. That evening, while the Palmer family is eating dinner. Leland sees Laura's half-heart necklace and questions her menacingly about her "lovers." Later, he goes into his daughter's bedroom and tells Laura he loves her. Laura wonders if he could truly be Bob.
Before bed, Laura hangs the painting on her wall and then falls asleep. She dreams about entering the Black Lodge. Suddenly, Cooper and the Man from Another Place appear in her dream. The Man from Another Place tells Cooper that he is "the arm" and offers Teresa's ring to Laura, but Cooper tells her not to take it. Laura finds Annie Blackburn (Heather Graham) next to her in bed, covered in blood. Annie tells Laura to write in her diary that "the good Dale" is trapped in the Lodge and can't leave. Laura sees the ring in her hand and is frightened. After looking outside her bedroom door, Laura finds herself inside the painting.
Laura awakens in the morning with the ring gone from her hand. Disturbed, she removes the painting from her wall. Meanwhile, Bobby, Leo, and Jacques Renault discuss drug scores. Jacques agrees to send someone to meet with him. That same evening, Laura prepares to go to the Bang Bang Bar where she encounters the Log Lady. Inside the bar, Jacques introduces Laura to two men. The group is about to leave for the Pink Room to have sex, but Donna shows up. The men are impressed with her "audition kiss" and they allow her to come along.
In the Pink Room, Laura discusses Teresa Banks' murder with Ronette Pulaski (Phoebe Augustine). She then engages in oral sex with the men. Laura becomes distraught when she sees a topless Donna making out with one of the men. She takes Donna home and in the morning, Laura tells Donna she does not want Donna to become like her. Leland arrives to take Laura back home. On the way home, Mike (Al Strobel), the one-armed man, shouts at Leland that "the thread will be torn" and shows Laura Teresa's ring, but Leland and Laura become frightened and drive away.
Leland pulls into a gas station parking lot to gather his wits, and recalls his affair with Teresa. He had asked Teresa to set up a foursome and invite some of her friends, but fled when he discovered Laura was among them. Teresa realized who he was and plotted to blackmail him and he killed her to prevent his secrets from being revealed. Laura realizes that Mike's ring was the same one from her dream, and was also worn by Teresa.
The next night, Laura and Bobby take cocaine in the woods, and wait for Jacques' drug messenger, the sheriff's deputy from Deer Meadow. The messenger delivers the drugs and takes out a gun, but Bobby shoots him first. He hastily tries to bury the messenger as Laura laughs maniacally, high on drugs. The following night, Bob comes through Laura's window and begins having sex with her. She asks Bob repeatedly who he is and then suddenly realizes that he is Leland, and warns Leland away from her the next morning. Upset, Laura uses more cocaine and has trouble concentrating at school. Bobby attempts to initiate sex with her, but Laura refuses. Bobby realizes Laura was only using him for his access to cocaine. He hands her his stash, breaking off their relationship. The angel in Laura's painting disappears.
In the woods, Laura tells James that "his Laura" is gone. She tells him that she loves him and runs into the woods where she meets Ronette, Jacques, and Leo. They hold an orgy in Jacques' cabin as Leland watches from outside. Leland attacks Jacques outside, and Leo flees in panic. Leland takes Laura and Ronette, both bound, to the train car. Meanwhile, Mike realizes that Bob/Leland is about to kill again and chases after him. In the train car, Laura asks Leland if he is going to kill her. She looks into a mirror and screams after seeing her reflection turn into Bob, who says that he wants her. Meanwhile, Ronette prays for rescue but breaks down because she is "dirty." Suddenly, an angel appears in the train car. Ronette finds herself untied and opens the door to let Mike in, but Leland kicks her out of the train car. Mike manages to throw in the ring. Laura puts it on, which prevents Bob from possessing her. Enraged, Bob stabs Laura to death.
Bob/Leland places Laura's body in the lake. As her corpse drifts away, Bob/Leland enters the Black Lodge, where he encounters Mike and the Man from Another Place who tell Bob they want all their garmonbozia ("pain and sorrow"). Bob pulls the blood from Leland's wound and flings it to the floor. Laura's body washes up on the lake shore, where it is found by the Sheriff's department the following morning. Laura's spirit later sits in the Black Lodge and notices Agent Cooper at her side. He places a comforting hand on her shoulder. Laura sees an angel appear and begins to cry and then laugh.

Question: Why did Leland kill Teresa
Answer: To prevent his secrets from being revealed.; To prevent his secrets from being revealed.