Heh. As of now we know little for Itachi's motivations. Lets just assume that because Sasuke saw Itachi slaughter thier clan, because he's an S-ranked missing nin in the "bingo book", because we know he in league with several other ninjas in an organization which contains several missing nins -- Itachi isn't the nicest person around.

I'll admit the whole "lets slaughter my entire clan" thing doesn't sound right. So theres a possibility there is some kind of conspiracy that concerns Itachi.