August Underground.

Imagine walking down the street and finding an unmarked VHS tape. Curiosity piqued, you take it home and pop it in. What starts off as two men screwing around with a video camera quickly transforms into an ultra-realistic torture sequence where the unidentified psychopaths tape their exploits as they torment and violate a woman tied to a chair. The devastation of your morals continues throughout the entirety of the footage, while subtly revealing the killer's shattered pasts via the interaction with their victims and each other.