The Man who shot John Lennon

The Man who shot John Lennon is a Frontline documentary for PBS released in 1988. The film tells the story of Mark David Chapman who on December 8 1980 shot the Beatles front man John Lennon in New York City. The documentary covers Chapman's history leading up to that winter night when he calmly asked musician Lennon for an autograph as he left The Dakota apartment building then waited for him to return before shooting him. Why did Chapman shoot Lennon and what was the connection of the book The Catcher in the Rye ?