I don't think Itachi is after Sasuke's MS because if Sasuke coincidentally got MS because of Itachi continously mocking him. Sasuke would pwn Itachi and even if he couldn't I don't think Sasuke would let Itachi take his eyes.
I believe that Itachi may have killed his clan as a test. Not to sure if it was mentioned in the anime.
and this test may have been hard for him because it was his clan he had to kill. And in order for him to gain the courage to do so he killed his best friend. Then he made himself believe that his clan was pathetic and killed them in cold blood to make sure he had no regret about it later. And he may have spared Sasuke so he could one day kill him and kill the leader of the Akatsuki.