Ending (SPOILERS)
So, what do you think happened?
Did Lenny kill his own wife with the insulin shots and make up the Sammy character? Or did she actually die in the attack?
You won't find one simple answer as many disagree.
Some will tell you that Leonard killed his wife, that John/'Teddy' was telling the truth. Often this entails that Leonard somehow 'remembers killing his wife',feels guilty, and transforms his own story into Sammy's (which is impossible if his condition is as described.)
Others will say that John lied and that Leonard knows his wife was definitely not diabetic. since Leonard had no memory after the attack, he would not need to create a fictional story about Sammy. (others will say that a flash of a needle in his mind proves Leonard does remember giving his wife an insulin shot. But Leonard dismisses it as untrue.)
Leonard seems to remember the past accurately, except for the attack itself. This is demonstrated by his waking up by the escort slamming the door. Leonard awakes and feels that his wife is alive and has just left the warm side of the bed.
If John tells the truth, then why lie about being a snitch? Why does he not be a cop when dealing with Dodd or the tatoo parlor woman?
Also, the accuracy of all of Leonard's tattoos must be questioned including the very first one about John G.
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I think his end speech all but confirms that he made up the Sammy character and wants to continue living a lie. Also, when Teddy is listing the things Lenny lies to himself about he says "your wife not surviving the attack," (as if she did live), & that he believed and tried to help him. However; the blow to the head left him how he is, and he at some point accidentally killed his wife.
The not remembering that his wife was diabetic was either an oversight or something he had deeply repressed.