Just another random thought, perhaps tobi used izanami on itachi, and thats what caused his illness that eventually killed him. ive always felt that his disease came without much explanation other than "hey shit happens"
It would also explain Itachi's knowledge of the jutsu, Tobi has a large amount of spare sharingan following the massacre so to lose the light in the eye wouldnt have been a problem and Tobi was well aware that Itachi was intelligant and impossible to control.