In the fight against Haku ninetails took over Naruto.

Right now, Naruto still has no control over the ninetails. The demon just gives his chakra to Naruto to use. He lends him his chakra the same way gamabunta said "i could lend him some of my chakra" after this day of running wild in the forest. The demon does this on purpose and as a self-protection meassurement.

If someone were to kill for example Sakura in front of Narutos eyes, chances are very good that the amount of anger will unleash the demon again.