How did this all begin?

At first I wondered how a movie going in reverse could keep suspense. Each time he flashes back to the next memory sequence he fills in another piece of the puzzle for us. Oddly enough the more he answers some questions the more confused I began to get. Where does this story start or rather how far will we go back? He always spoke about how he could only trust his handwriting. The problem with that is his handwriting is very generic and most likely easily reproduced. Also if I remember correctly the picture of Natalie has something crossed out and I keep wondering what used to be there. Was that of his own doing or did someone else cross that out. People don’t really have their signature way of crossing things out.  Regardless of that idea I want to find out if he killed the right guy or what the purpose of going backwards is. Obviously if everything worked out perfectly the movie would be going in chronological order though we would not feel the same way he does each time he forgets. Leonard also brings up an interesting idea that how can someone that can't remember even enjoy revenge? I feel like he is just a puppet remembering what someone wants him to remember. Once he kills his wife's supposed killer does someone simply tell him someone else killed his wife. It seems like all he remembers is a masked man killed his wife and that could be anyone. This will be a good question once we finish the movie, but is this the kind of movie that you only enjoy once because then it loses it effect once you know the mystery?