Sorry, but that answer doesn't make any sense, especially the memento part.
If it was just to remind him that he is in imminent danger, then just leave 2 or 3 bullets.
Leonard left evidence that he took a gun with just one bullet.
He intended to kill one person only.
The real interesting part is that Leonard has become a cold assassin.
He doesn't shoot Jimmy, he winds up strangling him, after attempting to kill him with a crowbar.
If one pulls a gun and sees only empty chambers, you have to shot to kill, rather than shoot out his kneecaps and other parts first. I view this as the opposite of the "make him beg" vengefulness that Leonard thinks of himself.
Also, if Leonard had to retreat back to his truck, the bullets would remind him that he has a gun on him that's loaded.
(works better than a camera)
The irony is that the bullet was intended for the second attacker, for John G, which we know has nothing to do with Jimmy.
And in the end, he actually uses it to kill John 'Teddy' G.