The Texas Chainsaw Massacre 3D

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Picking up after the events of the original film, the people of the small town of Newt, Texas, led by the corrupt Mayor Burt Hartman (Paul Rae), burn down the farmhouse of the Sawyer family for their role in aiding Jedidiah "Jed" Sawyer (Dan Yeager), also known as "Leatherface", in several murders. The arsonists are celebrated as heroes in the community, and the entire family is presumed dead. However, an infant, Edith Sawyer, is found by one of the townsmen, Gavin Miller (David Born), who quickly murders her mother, Loretta Sawyer (Dodie Brown), and adopts Edith with his wife Arlene (Sue Rock). Thirty-nine years later, Edith has been raised by Gavin and Arlene as Heather Miller (Alexandra Daddario). One day, Heather is notified that her estranged grandmother, Verna Carson (Marilyn Burns), has died and left everything to her. Discovering that she was adopted, Heather, her boyfriend Ryan (Trey Songz), and their friends, Nikki (Tania Raymonde) and Kenny (Keram Malicki-SÃ¡nchez), travel to Newt to collect her inheritance. Along the way, the group picks up a hitchhiker named Darryl (Shaun Sipos).
When they arrive, the Sawyer family attorney, Farnsworth (Richard Riehle), gives her the keys to the family house along with a letter from Verna. Excited about the property she now owns, Heather and her friends look through the house, decide to stay the night, and immediately set off to buy supplies and food, trusting Darryl to stay behind and look after the house. Darryl begins looting it and believes he will find valuables in a locked room in the cellar. Upon entering the locked room, he is killed by Leatherface. Heather and her friends return and discover the house has been ransacked, but choose to let it go. As Kenny prepares dinner he finds the cellar, where Darryl was killed, and Leatherface impales him on a meat hook. Nikki tricks Ryan into having sex with her in the barn. While searching through a room upstairs, Heather finds a decomposing body upstairs and runs to find her friends, but she is knocked unconscious by Leatherface. She wakes up in Leatherface's room where she manages to escape to the graveyard while Leatherface slices Kenny in half with a chainsaw. Hearing the screams and the chainsaw, Ryan and Nikki come out the barn to find Leatherface. Nikki draws his attention to them, and they run in the barn to hide. As he goes after them, Heather gets free, gets their van and saves her friends.
In the ensuing chase, Leatherface saws through one of the van's tires, causing it to crash and subsequently kill Ryan. Heather escapes and makes her way into a carnival, but Leatherface escapes the police as they patrol the grounds. Sheriff Hooper (Thom Barry) realizes that Leatherface survived the fire and is still alive. Mayor Hartman sends Officer Marvin (James MacDonald) to the Sawyer house to kill Leatherface against Hooper's orders. While looking for Leatherface, Marvin is startled by Nikki and accidentally shoots her, before being killed by Leatherface himself. While at the station, Heather learns of what the townspeople did to her family and flees. She is soon caught by Mayor Hartman's son, Deputy Carl Hartman (Scott Eastwood), who ties her up, gags her at the long-abandoned Sawyer slaughterhouse. When he arrives Leatherface finds out that Heather is his cousin and frees her, but he is attacked by Mayor Hartman and Ollie (Ritchie Montgomery), the alleged accomplice to the Sawyer house arson. At first Heather begins to flee, but she hears her cousin being beaten and, realizing that Hartman and company still want to kill her while Leatherface demonstrably does not, she decides to help him, killing Ollie with a pitchfork and giving Leatherface his chainsaw, which he uses to force Mayor Hartman into a meat-grinder. Afterward, Heather and Leatherface return home, where Heather reads the letter from Verna which tells her Leatherface will protect her as long as she takes care of him. Realizing Leatherface is the only family she has left, and traumatized by her ordeal, Heather decides to stay with him. All the while, Leatherface non-verbally promises to love her and protect her like he tried with his family members before.
In a post-credits scene, Gavin and Arlene show up at the mansion to visit Heather, intending on greedily splitting her assets. As they wait in front of the door, Leatherface answers with his chainsaw in hand.

Question: Why does Leatherface free Heather ?
Answer: Leatherface finds out that Heather is his cousin and frees her.