I have a theory about Itachi. First, some background.

Tsukiyomi is the name of the attack Itachi uses on Kakashi and Sasuke to make them relive over and over horrible moments, which eventually causes mental damage and puts them in a coma-like state.

It's part of the 'mange sharingan' jutsu used by true sharingan successors.

Tsukiyomi appears to be a genjutsu which inflicts both mental and actual physical damage on the recipient: Kakashi collapses soon after being hit with it, and Sasuke falls almost immediately into a coma-state, with blood dripping from the corners of his mouth and his eyes eerily blank. (chapter 147)

Now, in the next chapter, 148, Kisame asks Itachi why he is retreating from Jiraiya. Itachi replies that there is no need to rush, that his body needs rest (implying that using the Tsukiyomi repeatedly takes a great deal of stamina/chakra) and also that:

"Not to mention, until Tsukiyomi becomes Amaterasu, we should not use it."

Now, the translation lists Tsukiyomi as 'moon' and Amaterasu as 'sun', thus "until the moon becomes the sun, we should not use it." However, Amaterasu is actually the sun goddess, the principal goddess in the pantheon of Japan's native religion, Shinto. The Imperial line was held to have descended from Amaterasu, hence why the Emperor is so revered.

What I think this means for Itachi: if Tsukiyomi is genjutsu so powerful that it causes both mental and physical damage and puts the user in a coma, and Amaterasu is a jutsu even more powerful, named after the highest goddess in Shinto myth, then Amaterasu must be a genjutsu that kills. What could be a more powerful jutsu than a way to kill someone only by locking their eyes with the sharingan?

No wonder Orochimaru wants Sasuke so badly.