Naruto said:
1. I won't die until I become Hokage.
2. I can't become Hokage if I can't save a friend.
3. (to Itachi): I will take care of Sasuke, without killing him.
There is no contradiction here. Naruto believes that Sasuke hasn't gone all bad and that he can save him without killing him.
His words to Sasuke:
4. (to Sasuke): If we fight each other, we die.
were a warning to Sasuke about the consequence of him continuing on his path, but also a commitment to him that he will stick with him and not give up on him, no matter what. These words carry a message of hope to Sasuke rather than a threat.