Sorry guys, but Leonard didn't kill his own wife the way Sammy did. Leonards wife died of an OD she took herself (there a short scene of her opening her eyes while she's lying on pillow. She's going "into a coma from which she never recovered"). She stole the drugs from a drugdealer after she discovered she could not only take advantage of Leonard by taking the insurance money that comes in (he was "well insured") after he got CRS (after he trew her out of the car after she refused his marriage proposal he "was hit from behind") but also by using him as an istant killer. She learns he's an instant killer when he kills her lover. She gets called of to work because some waitress cancelled ("Helen can't come"). He finds a picture in her wallet she forgot of her and some guy. This guy just walks into the room, asking for Catherine 'cause they're going on a date (Yeah I know you, You're that memory guy). He kills him with a bottle and wanders over his dead body until Catherine comes home (the movie basically is a comedy). Catherine takes him on his first killing streak, seducing drugdealers and calling Leonard in for help, claiming the drug dealer his raping her. She ODs on her last job and Leonard goes to the first mental hospital.
As far as Sammy goes: "Sammy didn't have a wife". Leonard "exposed him for what he was: a fraud" and he got "a big promotion". Leonard first remembers him to remember himself he's got CRS (the disease Sammy claimed to have). Then slowly he transmits nasty memories from his part onto Sammy's story. In the third and last mental hospital he 'mirrors' himself with Sammy until two satisfying characters remain: Sammy had a lovely wife who accidently ODs (Sammy did it, because Leonard did kill his wife. But it was his second wife, his first highschool love (there's a picture of her on the official website. she looks up from below. It's cut out of a senior prom highscool picture. She uses it to make Leonard remember her. She also makes Leonard remember his wife is dead (the short story refers to this quit clearly - the picture on the graveyard)). That's how she gets him out of the second mental hospital. The doctors believe he has 'cured' and stopped looking for his dead wife. But Natalie and Leonard have sex, she takes a shower, Leonard (half 'resetted' from the sex) 'wakes up' and hears somebody in the bathroom. Thinking it is John G he runs in and strangles her with the shower curtain over her face. After realizing he's strangled his own wife (with her last breath she sucks the curtain against her face and Leonard sees he's strangling a woman) he goes to the third mental hospital. There he makes up the story of Sammy who accidently killed his own wife and him still looking for John G, the one that got away, after realizing the only time in life he was really happy was when he was chasing the alleged killer of his wife (this is the famous 'so how can I heal'-scene. I've explained this one in an earlier reply). He creates his own foe (his fifth and final 'friend').
The whole story is explained (in steno) in my post 'The true chronological story of Memento'. I've just explained only part of the story and given only a part of the evidence available (in the movie, the short story and the website). And you probably still don't believe me. Maybe now you understand why I wrote the story in steno (although I have explained quit a bit in earlier replies).
But believe me, if you do look up THE FRAME, it is REALLY the true story...