Why did such a promising and seemingly kind (see episode 129) ninja acquire such a lust for power? How did he become so cruel? Sure, everyone has their theories. But Masashi Kishimoto has neglected to explain, thus far, the true reason Uchiha Itachi became a homicidal psycho.
Or has he? I believe the answers to all of these questions are hidden within episodes 129 and 130 of the anime (I haven't read the manga), for those who would look for them.
If Itachi had died before he had the chance to become a psychopath (yeah right, he's, like, invincible), the epitaph on his grave would have said something like this: Uchiha Itachi: Beloved son, brother, and skillful Konoha ninja. Everyone looked up to him, and he became a standard to aim for in the Uchiha clan, especially for Sasuke.
Before I go any farther into this analysis, I must use a real-life genius to illustrate my point. Take INSERT FAMOUS SCIENTIST HERE, for example. The man/woman's brilliant, I'll give him/her that. But it seems to me that he/she misses out on a lot of things in life, being so absorbed in his/her research and all. Not that that's bad! But in becoming so absorbed with certain things, in his/her case, something he/she excels at, he/she tends to block out other people. And thus, one could conclude that he/she is not particularly interested in those around him/her. He/she may care about them, but he/she is not particularly interested.
And so it is with Itachi. Like his father said, "He's a little different. He's the kind of person who doesn't like to interact with other people." And as Itachi himself said, "This is what happens when you are skilled. Power causes you to become isolated and arrogant." One can guess that this is because of how absorbed he is in his talent, not himself, but his talent. As time passed he came to truly realize just how exceptional he was. "Hey!" he probably thought to himself. "I'm more powerful than my best friend Shisui; therefore I can kick his butt and obtain the Mangekyou Sharingan. SWEET!" So he goes and kicks Shisui's butt and obtains the Mangekyou Sharingan. But he hasn't had enough fun yet. Oh, no. He decides, "Since I totally pwn everyone in this pathetic little village, why shouldn't I go kill the whole clan?" Long story short, he did, and for exactly the reason he stated on the show. To see what he was capable of. He has come to realize his personal philosophy on life: I already know I'm better than everyone else, but why don't I go and pwn them all, just to make sure? And he will let nothing stand in his way.
So, in a nutshell, Itachi became bored with simply being the best in his village and decided that he should be the best in the world.
a note from the desk of ethereal damsel
Now that you've finished reading my thoughts on Itachi's insanity, please review and give me your own theories on this subject.
Disclaimer: I don't own Naruto.