Itachi never told naruto that he better not let sasuke die.
What Itachi wanted was for Naruto to bring sasuke back from the darkside.
I dont think Itachi would care if sasuke dies...just as long as he dies trying to do the right thing.
I mean come on now..Everyones always asking how sasuke is going to redeem himself and stuff
But if he dies...he doesnt have to.