Orochimaru probably didn't have much to do with killing the Uchiha, he's probably not powerful enough. Itachi probably also would've just killed him, and they wouldn't have been able to be in Akatsuki together. Someone else killing the Uchiha and Itachi coming at the end and seeing it could make sense. Then Itachi told Sasuke to seek power and revenge so he could become stronger and be the one to avenge the Uchiha, since Itachi already knew his max potential wasn't as great as Sasuke's and wouldn't be enough to defeat the person. It's hard to come up with a decent theory that goes against what's already been said (Itachi killed the Uchiha), with only the few clues about the clan massacre. We'll probably see the truth when Sasuke becomes a more worthy opponent and faces Itachi again.