Leonard Gregory Kastle (11 February 1929 – May 18, 2011) was an opera composer, librettist, and director, though he is best known as the writer/director of The Honeymoon Killers, his only venture into the cinema, for which he did all his own research. He was educated at the Curtis Institute of Music studying under opera composer Gian Carlo Menotti and also the Juilliard School. He was an adjunct member of the SUNY Albany music faculty. His operas include Deseret (1961), on a libretto by Anne Howard Bailey about Brigham Young, which he directed for the NBC Opera Theatre. He wrote in a Romantic style that he believed the music academy did not hold in high regard as something for contemporary composers to do. His favorite composer was Gustav Mahler, whom he believed was not at all respected when he was in music school. In 1956, Kastle composed a thirteen-minute "made-to-measure" opera, titled The Swing, for two singers, a speaking part, and piano accompaniment. It was commissioned by and broadcast on the NBC television network on Sunday, June 10, 1956, at noon.

• The Honeymoon Killers (1970)