Oldboy (2003)

When tarrentino procaims a movie as a must watch, you know what youre gonna get to watch. A recent set of allegations of the mass murder in Virgina Tech point to the resemblence of some of the stances and scenes to this movie. Startling indeed, but the VT killer was a paraniod, delinquent foolish idiot comments on whom are not worthy.
The movie is shocking in the end moments as the viewer gets to unravel the mysteries of Dae-su Oh along with Dae-su Oh. A great movie to watch especially if you can watch someone eat a live octopus. (it actually is shown clenching to his cheeks while he's eating it. not something for the weak hearted)
The ending is deliberately ambiguous, and the audience is left with several questions. Try the movie.