Chapter 27 is a 2007 biographical drama film depicting the murder of John Lennon by Mark David Chapman. It was written and directed by Jarrett Schaefer, based on the book Let Me Take You Down by Jack Jones, produced by Robert Salerno, and stars Jared Leto as Chapman. The film takes place in December 1980, and is intended to be an exploration of Chapman's psyche. Its title is a reference to J. D. Salinger's novel The Catcher in the Rye, which has twenty-six chapters, and suggests a continuation of the book.

The real Mark David Chapman is currently incarcerated at Wende Correctional Facility, on a guilty plea. Aside from two interviews with Larry King and Barbara Walters, both in 1992, he has not spoken with the media. However, Chapman did reveal the mechanics of his unraveling during those three days in New York City to journalist Jack Jones. The interviews were published in 1992 as Let Me Take You Down: Inside the Mind of Mark David Chapman, a book of Chapman's recollections of his act of violence. Chapter 27 is based on this text. The title "Chapter 27" suggests a continuation of J. D. Salinger's novel The Catcher in the Rye, which has twenty-six chapters, and which Chapman was carrying when he shot John Lennon. Chapman was obsessed with the book, to the point of attempting to model his life after its protagonist, Holden Caulfield.  According to the British music magazine Mojo, the title was also inspired by Chapter 27 of Robert Rosen's book Nowhere Man: The Final Days of John Lennon (2000). Rosen's book explores the numerological meaning of the number 27, "the triple 9", a number of profound importance to John Lennon. Lennon was deeply interested in numerology, particularly Cheiro's Book of Numbers, along with nine and all its multiples. It was Chapman's goal, according to Rosen, to write Chapter 27 "in Lennon's blood".  Like Chapman, Schaefer is a fan of both The Beatles and J.D. Salinger's novel The Catcher in the Rye, and said he began the script to try to understand "how someone could be inspired to kill anyone as a result of being exposed to this kind of beautiful art. It really bothered me, because Lennon and Salinger have always made me feel so much better, and so much less alone."

Using a quote from the above article, answer the following question: why is he incarcerated?
he shot John Lennon.