It is said in the manga that Itachi's reason why he killed the whole Uchiha clan was because they were planning to take over Konoha, since they are well-known for being the most powerful group. Yes, he didn't kill Sasuke since he wanted him to be stronger and hate him. It's pretty odd really, since I didn't think it was that way... But it actually fit perfectly! What I don't understand is why he joined the Akatsuki in the first place...
