Horror movies depend a great deal on the element of surprise, unless you're Alfred Hitchcock. He was the master of letting you in on the secret early, then letting you watch people twist in the wind.
https://www.youtube.com/watch?v=EimClT1q-1M
The best "fright films" use music to great effect. While there have been some great themes written in the last 30 years, such as Tubular Bells - used in The Exorcist...
https://www.youtube.com/watch?v=8LEWkwvaNcs
...it is still hard to find any music better suited for setting a scary mood than classical music.
https://www.youtube.com/watch?v=Nnuq9PXbywA
With that in mind, here is a collection of 13 MORE Scary Pieces of Classical Music for Halloween courtesy of Limelight Magazine.