Kes going senile in her final years would've been more interesting in it's own right without the crazy time travel stuff. A teaser of Kes in her ship barely remembering Voyager except that they brought her pain or something would've really set a tone for the episode. But even with her memory going crazy I can't see her being violently vengeful. That's just way too out of character. Even in her strongest moments like in Warlord, sure she had an inner strength and resolve, but she wasn't just plain offing people.