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Leonard remembers everything up to the incident. So this means he knows about Sammy and his fake condition. This also means he remembers Sammy didn't have a wife.
So after the incident, how could he remember about the insulin.
He killed his wife with insulin only after the incident.
As with a lot of things in this movie, there is no clear answer. However, I can share my opinion.
Killing his own wife was an event so scarring and traumatic, that even through his condition, he couldn't just forget it ever happening. However, he also could never believe about himself that he had actually killed his wife. He wouldn't have been able to live with it, and his condition wouldn't let him remember. So his mind projected that event onto something that happened before he lost the ability to form new memories, meaning that he would still be able to remember it.
I'm sure this isn't the only answer, but this is what makes sense to me. The movie is dealing with trauma and the human brain here, so I'm not even quite sure if there's a definitive answer that's been researched/proven. You could do some research into trauma to see if Leonard's condition is even realistic in the first place. The movie probably took some creative liberties. I hope this helped.
Leonard remembers everything up to the incident
This is not fully correct, in the sense that Leonard is not shown/claimed to have perfect memory for everything that happend before the accident. His condition simply means that he doesn't remember things after the accident. From before the accident, his memory is as good as any average human's is. Not perfect, just normal, in comparison to his abnormal memory for things that happened after the accident.
Other than the amnesia, he simply suffers from a second mental condition: he has disassociated a traumatic part of his memories in order to not have to bear its weight.
Technically, these conditions exist individually, it just happens to be the case that he suffers from both of them at the same time.
