I dont think Sasuke is planning to kill Naruto again since he could have done so the last time they fought. It would be pointless for him to wait another 3 years when its even harder to kill Naruto now. Im pretty sure there's some other way besides "killing ur best frd". Or maybe there are different types of mangekyo and they can be acquired in different ways. Just like how Kakashi's is different from Itachi's.