These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Alternate Character Interpretation: A meta example, as the movie provides one of its own. Natalie, for the first viewing of the movie comes off as a manipulative, evil bitch, but on the second viewing all of her actions become immensely more justified when you realize that she knew from the beginning the protagonist had murdered her boyfriend.
Another is whether Lenny really killed his wife or it's just another of Teddy's lies is left up to the audience, as there's evidence for each. There's also whether or not Teddy being killed will stop his killing spree.
And probably the most poignant one. Is Lenny a victim, being manipulated by Teddy to be a hired gun, who frees himself from Teddy's influence at the end of the movie? Or is Lenny just a serial killer with anterograde amnesia who just wants a purpose in life, even if it means deluding himself? The second has more weight than it initially appears, if one wonders what kind of life Lenny will/can have if he gets away with it.
What about Teddy? Is he just a scheming crook who manipulated a hapless amnesiac into becoming his own personal thug? Or is he, in fact, the only real friend that Lenny has—a devoted pal who's been keeping Lenny out of jail, holding him back from becoming a full-blown serial killer, and giving him purpose and protection—all because he just wants the poor guy to be happy?
The Reveal: Subverted. Teddy hiding his identity as John G is a Foregone Conclusion about a fourth of the way through the film, with the rest of the movie leading up to that point. But he is not the first John G that Leonard has killed.
Teddy, knowing what he knows about the protagonist's facts concerning the case, and knowing that his own real name is John G., why he went within a hundred miles of this guy can at best be chalked up to greedy recklessness and less charitably just a death wish.
At least he used an alias. Still, it's a bad idea.