Conversation so far:
what was the development of the movie chapter 27?
Chapter 27 is based on the text Let Me Take You Down: Inside the Mind of Mark David Chapman, a book of Chapman's recollections of his act of violence.
what else was the movie Chapter 27inspired by besides a book?
The murder of John Lennon by Mark David Chapman.

Question: what was his biggest accomplishment?
Self-contained rewrite: what was John Lennon's biggest accomplishment?