Major Plot Hole
I love this film. I think it has a ton of depth. It works on many levels and if you wish to pull back the layers of that onion, you'll discover it still works...except for one scene...
Let's start from the premise revealed at the end: Leonard's abbreviated short-term memory is essentially following a bread-crumb trail of lies he leaves himself, to the resolution he 'wants'.
But, he doesn't know what he wants, because he doesn't know how it all began, because he's done everything in his power to obfuscate this, and Teddy has assisted him.
Yet...
In the diner, Natalie asks him what he will do when he finds John G and murders him. Leonard's response is, 'take a picture so I remember, I suppose', or something along those lines.
Leonard would just kill himself at that point, right? He has no reason to exist after he kills John G. It wouldn't even take a moment of thought.
Why would he take a picture to remember when he kills John G? Was Natalie calling *beep* on Lenny? Did Lenny reveal some *beep* After Lenny guns down John G, that gun is going straight in his mouth, right?