If you think about it, he wasn't the least bit surprised at Sasuke's answer, and overall pretty collective throughout the whole ordeal, while Kakashi, Sakura and the TB were gasping for air out of shock. Maybe it had something to do with him sensing feelings, and this was something he foresaw based on Sasuke's. He knew Sasuke had a hidden motive, but was waiting for him to bright it to light to take proper action. The big *sigh* and "The sibling rivalry will end here!" response spoke it all, Naruto has the resolve to put Sasuke down, and was prepared to If need be.