I don't think I buy that Ammits endgame is to kill everyone, we have already seen a person being judged and found "good" and allowed to live, so its not like Ammit thinks everyone needs to be wiped out.
Well, for now, anyway. But, you figure that Ammit will keep an even, steady keel... forever?
But, even so - unless Ammit can actually see the one and only future for each person, that means that innocents will be killed in the name of maybe. And that's not really acceptable.