i was just rereadinsome old chapter and i came across this:
Naruto 309 | Read naruto chapter 309 manga online
Sasuke is talking with the nine-tailed fox and after sasuke suppresses his chakra, the nine-tailed says "Don't Kill Naruto or you'll regret it."
My theory is that the fox knows that madara is still alive and if naruto dies, then the fox's power will be released and madara will come back in power by harnesting the foxes power. it's not fully thought out and when i get it all together i will edit but i wanted some feedback before i really get into it.