In reading over the original story and the photographer's rebuttal, frankly I don't see how his photo is relevant to the question of gun violence - it depicts a man holding a shotgun. He's not pointing it at the photographer, or firing it in any direction... to me, that's like taking a photo of me holding my circular saw and saying it is about injuries in the workplace. I'd buy the argument if it showed the subject in a room full of guns.
Perhaps part of the argument between the subject and the photographer over calling him a 'sniper' is something that gets lost in a cultural divide between an Italian civilian and a US Marine. Having worked for the Marines, I know that they are EXTREMELY particular about anything and everything relating to service in the Corps, from uniforms to job specialties. So while it may not have meant much to the photographer, it means a lot more to the Marine.
I just have a general problem with the whole concept of this "story" in the first place, as I don't really see it as documentary. It seems no more journalistic than Richard Avedon's American West series.