Memento ending explained: Who was behind the mu*der?

Memento is a complex film due to its fractured narrative that forces the viewers to ponder what they saw on screen. The second directorial feature of Christopher Nolan (also written by him) was based on his brother Jonathan's short story Memento Mori, and it was released on March 16, 2001. Here's the official synopsis:

A former insurance investigator who now suffers from anterograde amnesia uses notes and tattoos to hunt down his wife's murderer.

The psychological thriller film stars Guy Pearce, Carrie-Anne Moss, as well as Joe Pantoliano in pivotal roles. The film is told in a backward structure and consists of black and white scenes to separate the timeline of different events.

What is the plot of Memento?

Memento revolves around Leonard Shelby (Guy Pearce), who used to work as an insurance investigator. He suffers from anterograde amnesia, which leads him to have short-term memories which happened when someone killed his wife and attacked him. Leonardo uses his body tattooed and photographs to track the killer. Slowly, viewers start to realize that the narrator is not reliable and the events are out of sequence.

How does Memento end?

Towards the end of Memento, Leonard finally manages to track John G, his wife's killer. He even manages to eliminate John, only to realize that the person had passed away a year before the death of his wife. His short-term memory has made him immune to the truth, as he is unable to remember clearly what exactly happened. Leonard quickly realizes that people around him have been using his memory loss problem for their own benefit. His wife had died due to excessive use of insulin (she was diabetic), as Leonard could not remember the doses properly. Despite figuring this out, he ignores everything and continues to chase the truth that relies only on his tattoos and notes.

Teddy, meanwhile, was a cop investigating the death of Leonard's wife, Catherine. He helps Leonard track and kill the second guy, but that is not enough for the protagonist. Teddy then confesses to using Leonard's condition to eliminate several men. The Sammy Jankis story was running parallel with the main story and showed a man. It turns out that the man was a fraud, and it was actually Leonard's story. (Sammy's wife had diabetes and did not believe in his memory loss condition. This led to his wife dying of an insulin overdose.)

Memento essentially touches upon the unreliability of human memory. It showcases how we are at the mercy of our perception, which is further reliant on our surroundings (in the film's case, the tattoos and notes). Leonard finds out the truth, but still continues to chase someone because that gives him a purpose. While the film's ending is a bit ambiguous, multiple viewings will let you peel off certain layers from the narrative.

Christopher Nolan spoke about the big reveal in the film and said,

“The most interesting part for me is that audiences seem very unwilling to believe the stuff that Teddy says at the end – and yet why? I think it’s because people have spent the entire film looking at Leonard’s photograph of Teddy, with the caption: ‘Don’t believe his lies.’ That image really stays in people’s heads, and they still prefer to trust that image even after we make it very clear that Leonard’s visual recollection is completely questionable.”

What do you think of the film's ending?