Wendy Carlos is credited as Walter Carlos in A Clockwork Orange because she scored the film before undergoing sex-reassignment surgery in 1972. Carlos is known for using synthesizers (instrument of choice in the 1980s) to play classical music, making her the perfect composer for Orange, a film about a punk obsessed with Beethoven. Her synth tracks were ahead of her time, which enhanced the film's near-futuristic feel.
Two pieces of music are critical to the plot. Alex is sensitized to hate Beethoven's Ninth Symphony. Music can be powerful like that, and Carlos often uses music to make the viewer feel uncomfortable. Plus, the film makes use of generally happy, popular songs, like "Singin' in the Rain" during scenes of violence and rape. You'll never look at that cheery musical the same way after watching A Clockwork Orange.
Carlos worked with Kubrick again in 1980, and also scored films for a company more creepy and disturbed than even Kubrick could ever imagine: Disney.