A lot of films use classical music in their soundtracks, but horror films tend to use it in a unique way. Most of the time, the beautiful music is creating contrast to the horrible images on the screen. Other times, the classical music serves to unnerve the audience and enhance the disturbing scenes. Here are some films that use classical music in awesome ways.
1) Battle Royale
This film uses Verdi's dramatic Dies Irae for the intro of the film where the Battle Royale program (where one class out of all of Japan is chosen to fight to the death on an island) is being introduced and the winner from the previous year is seen, covered in blood and smiling eerily.
It also uses a Schubert Lied (or art song) call Auf dem Wasser Zu Singen. This beautiful song backs the tragic story of Chigusa Takako, a runner who finds out the boy she likes also likes her back just as she is dying.
This film is about cannibals in California during the Mexican-American war. The song Hail Columbia, which was considered the US unofficial national anthem before 1931, accompanies a scene that should be fairly normal: a group of soldiers eating a celebratory steak dinner. However, the eating and the meat look so grotesque and disgusting, it's stomach churning. Hail Columbia in the background is at first played normally, but as the scene gets more disturbing, the balance falls apart and it's played just as grotesque as the scene. (I unfortunately couldn't find the clip and the music sounds fairly normal on the actual soundtrack, but watch the film to hear it!)
Oldboy is a complex revenge film. The Allegro non molto movement of Winter from Vivaldi's Four Seasons is used to accompany a fight scene and the following torture scene (not for the faint of heart). The intensity and speed of the music enhances the feeling of the scene, but I wouldn't think such a beautiful piece would go with such a gruesome scene.
4) The Exorcist
The Exorcist is an interesting and disturbing film of a young woman possessed by demons (or just becoming a woman if you read it the way I do). An equally disturbing piece of music is used to enhance the feel of the movie: George Crumb's Night of the Electric Insects from the Black Angel's string quartet. It sends chills down my spine with no visuals at all.