Was Mark David Chapman a model for Rupert Pupkin?
As a teenager, Chapman revered The Beatles and fantasized about being a member of the band. When it finally dawned on him that it wasn't going to happen, his obsessive worship turned into obsessive bitterness and hatred, much as Pupkin's "reverence" for Jerry turned to violent hostility:
http://www.cnn.com/2015/12/08/us/mark-david-chapman-lennon-interviews/index.html
I wonder if Chapman was a model for Pupkin, in the same way as Arthur Bremer was partly the inspiration for Travis Bickle.