Young Guns 2

Billy the Kid (Emilio Estevez) gets caught off guard without his gunbelt by Pat Garrett (William L. Petersen).  It's implied Pat Garrett shoots and kills Billy. 
The movie flashes forward to the middle of the 20th century to Brushy Bill Roberts, who claims to be Billy the Kid, and a young historian.  Roberts has been narrating the movie from the beginning.  He claims Pat Garrett didn't kill him and was able to slip away and live peacefully. 

Thanks DrummerBrian!