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As far as I know it's just the first person who remembers a victim that dies next, everyone else remembering them later is spared.
So perhaps John did remember the others (or at least some of them) after Katherine dies (and just forgot about Katherine herself).
Who's to say how much time actually passed between 24 and 25? The recordings just cover the times when a death happens, there's no actual time given - so John could theoretically have wandered through the whole house, figuring out everything in the space between 24 and 25.
Perhaps he thought that he, as the only person left alive in a house full of dead bodies, a stranger to top it off, he would be found guilty of their murder and committed suicide in a panic?
OR he figured out the curse after all and killed himself to try to stop it from spreading.