It's been boiling in my mind for a while, what possible reason would Itachi have for keeping Sasuke alive? More so, why would Itachi want Sasuke to get stronger? It doesn't make sense for Itachi to risk his life by letting his younger brother attempt to kill him?
I've been thinking, if Itachi gained the Mangkou Sharingan (sp?) for killing his best friend... maybe there's a greater Mangkou Sharingan for killing his younger brother? It seems to make sense, Itachi gained his Mangkou Sharingan by sacrificing his best friend. His best friend was close, but was he closer than his own blood brother?
Could an even greater sacrifice allow Itachi access to an even more powerful Sharingan?
Yes, Itachi killed the rest of the Uchihas but he wasn't really close to them (don't know about his parents). Maybe Itachi was already trying to gain the Greater upgrade by trying to kill everyone left of the Uchiha but realised they were all too weak by the time he reached Sasuke?