"Prisoners" (rated R) 

 "Prisoners" is a film about two children who get kidnaped. The father of one child is looking for the man responsible for the kidnaping. When the police catch one man that they think is responsible, but can't prove anything, they let him go. The father is furious about the release of the suspect and takes it upon himself to prove this man guilty. The father kidnaps the supposed  kidnapper and begins to torture him. The police say that the suspect has the IQ of a 10 year old and doesn't know anything, but the father doesn't believe them and goes as far as necessary including waterboarding and locking the man in a small room with know light and barely enough room to sit. 

here is a clip

  How it ends is the man with an IQ of a ten year olds aunt who he lives with is the kidnapper and he was kidnapped and drugged. When the aunt kidnaps children she makes them do a series of mazes, and if the complete them the get to go free. But the catch is that the last one is impossible.