There's part at the start of manga (part 2) that mystified me ever seen I saw it.
Basically the fox is saying to Sasuke he'll end up regretting it if he kills Naruto.
So what can we conclude from this:
1) Is it a bluff to stop or scare Sasake from killing Naruto?
Sasuke was stronger than Naruto at that point in time.
2) Do you think the fox knows something the sharingan users don't, now that he is sealed in Naruto?
I think the fight between Madara and Naruto, the Fox will reveal all.
As Itachi's gift clearly is concerned with Sasake and that power will come into play then.