If you kind of read into what itachi was to talking to Sasuke and said to the cop's, was that the clan was becoming stagnent and to complacient with the life they had come to regard as normal. I think Itachi killed the rest of the clan to ride it of the stagnation that had formed over the years. He left Sasuke because he knew that his little brouther had a lot of potentional it not more than itachi himself and would restor the Uchiha clan to former glory it once had.