I believe that itachi has more background than what we think , all that about killing dozens of ppl only to prove strengh or to break clan bounds makes little sense, if itachi was some kind of gaara maybe i would undestand.

This is completely my invention but i think that itachi killed all the clan following orders and that he agreed to it to save at least 1 member of his family. Maybe the one who spoke to him about mangekyo, the other uchiha member threathened him about killing all the clan or cheated him, itachi is weaker than the other member and thats why he had no more option than to do it, maybe the 3rd magekyo killed the clan and itachi couldnt do anything, or was frigthened, or he was allied with him or under his command.

He left sasuke alive because he knows that sasuke will become stronger than him and could be able to kill the 3rd mangekyuo user for him, and tries to make sasuke to improve with hatred. Quite stupid i know but well its my spare time and the explanation of killing everybody for no reason seems equal absurd. In the end sasuke will fight itachi and beat him but spare his life in the last moment, then 3rd magejyo user will kill itachi (or tell sasuke the truth that he killed all uchihas) and sasuke will make an stupid power boost to beat him down.

bye.