Naruto has this Onizuka-like faith and naivety in his belief of Sasuke's goodness prevailing in the end. I can totally see a scenario where Itachi is down, sasuke is going to strike a deathblow, and naruto throws down an ultimatum of "don't do it, don't kill him".
What would totally twist is if sasuke goes emo-cry, stands down, then itachi attacks and mortally wounds-kills-maims sasuke. Leading naruto to a full blown kyuubi release, all 9 tails come out. (is going on a wild speculation tirade now)