context: A farmer kills three male students of an exclusive and elitist boarding school called Hallestad. Afterwards, the farmer kills himself by drowning in the river nearby. His body was never found.99 years afterwards, a female student, Rebecca (Sasa Bjurling), is depressed and commits suicide jumping from Hallestad's roof.A year later, life goes on at Hallestad. Sara (Rebecka Hemse) is making a class project about the legend of the farmer. 2 new students also arrive at Hellestad, Felix (Jesper Salén) and Leo (Peter Eggers). Leo immediately hits it off wit Sara, and Felix with Therese (Jenny Ulving), Sara's best friend and roommate. As the date of the murders and suicide arrives, Mans (Daniel Larsson), one of his friends and his new girlfriend Amanda (Rebecka Ferguson) prepare a party at the old barn where the farmer killed the 3 boys.Also, Rebecca's father (Tommy Andersson) runs away from the psychiatric hospital where he's been put away, killing the institution's new doctor. He goes to Hellestad to find Rebecca, who he refuses to believe it's dead. He enters Sara's room in the night, frightening him. Then, he talks to someone disguised as a scarecrow in the old farmer's barn.Sara talks to Anna-Lisa (Bojan Westin), the granddaughter of the farmer. She gives her many documents from the time: photographs and her father's diary. The farmer had killed the 3 boys because they had raped and killed the farmer's daughter. Nobody believed Anna-Lisa's father, who saw it all. One of the three boys was a relative of Mans' father (Kjell Bergqvist), who sits at the boarding of directors and finances the school.Headmaster Laia (Kerstin Steinbach) threatens to expell Professor Ek (Stig Engström) if he doesn't stop Sara's investigation. Ek doesn't fall for it and encourages Sara some more.Somebody dressed as a scarecrow kills one student who was supposed to leave for London to his father's funeral. Mans' father is also killed, in the middle of a road. He had to stop to remove a scarecrow from his way.Sara watches an old film concerning Rebecca's birthday party as a child. He thinks she saw something strange on it, but is interrupted by Prof. Ek. The night of the party arrives.Sara thinks it's Leo, because she found Rebecca's postcards addressed to him. [Shouldn't it be his reply to her?]
  and a scarecrow kit with a stained knife under his bed. Mans and Amanda laugh about Sara's abortion of Mans' child. Mans' thug has tried to rape Therese, being completely drunk. Therese and Felix will arrive from the party because they have heard Sara shouting "help". The clerk is dead, as well as Mans and his thug. Sara puts Leo in a room, when she realises it has been Felix all along. He was Rebecca's brother, but was sent to Mexico as a child because he was a danger to her. Now he's come to avenger her, and the first person he killed was his and Rebecca's father.Therese and her escape through a basement window. Therese falls an old sewage well and gets hurt. Sara goes down trying to help her. They fall sleep for a little time until Felix comes after them. They ran through the sewage, but Felix traps and kills Therese. Sara gets into an old boat, but can't get far, because the engine is dead. There, Felix attacks her.Prof. Ek comes back to Hellestad to quit his job, but finds Laia muttering her farwell speech over and over again with Man's father's head in a bag. She's gone crazy forever.Sara hits Felix in his head with an anchor, saving herself. However, when the police arrive, they only find the farmer's skeleton.8 months later, in Stockholm, Sara and Leo are celebrating within a party/demonstration. She thinks she saw a board from Felix, but a second later it's not there.
question: Why did the farmer kill the students?
answer: they had raped and killed the farmer's daughter