Why couldn't Naruto just let Sasuke go?

Even with the power of the Nine-Tailed Fox, Naruto can't beat Sasuke because Sasuke is stronger than Naruto from the start. At the Final Valley, Naruto couldn't beat Sasuke even with the one-tailed transformation. In part two, Sasuke have managed to become one of the Uchiha clan who can really control the real power of the Sharingan. Even the Nine-tailed Fox admitted that Sasuke's chakra is more ominous than its own and he was like Madara Uchiha.