Why did Itachi killed the whole clan?

Itachi eliminated his whole clan for one reason: To gain absolute command and power thru his Sharingan. His Mangekyu Sharingan is a further advanced version of the normal Sharingan. And it needs a 'special requirement' for it to activate (as mentioned by his Dad), that is, to kill all people close to you. Itachi first killed off his bestfriend (I forgot his name, but those Uchiha regulators mentioned it. he killed him thru drowning), then continued to kill off all people related to him to complete the Mangekyu Sharingan. Why did he leave Sasuke to live, he wanted someone to test his skills with. He said this because he knows that Sasuke can also activate his own Mangekyu Sharingan. Sasuke, after finding out that Naruto is better than him, joined with Orochimaru to gain power thru his cursed seal AND thru his Sharingan. Sasuke intends to activate his Mangekyu Sharingan, which means he has to kill off some,if not all, of his friends. Look out for Naruto's 2nd season next week. =)

Frankly i think he went crazy in the head but i think he got fed up having to live up to expectations of the uchiha clan. Then again, it could be because he was testing out his new sharingan. We'll probably not know till he battles Sasuke.

It's still a mystery but if i remember correctly he said something about testing or measuring his capacity and that he had no hope for the clan. It might be something else but we can only guess.

WOW guys. i've learned a lot!! so that means itachi joined akatsuki way back. cant wait to watch the fight between the brothers.....

To spoil the beans, its because akatsuki's orders was for him to prove his loyalty to the organization. And the one thing that he pulled up was to eliminate his clan...

Its been a while since i seen it, but i think he just got fed up. This is because his father and other members of the clan had really high expectations for him and he got fed up with trying to fill those expectations. I think he mentiones it when he is being questioned by 3 of the police members i think. Correct me anyone if I am wrong..hope this helps..

Yup! I agree that he want to test his strength and know his limitation. It was said on the series that in order to obtain the mangekyu sharingan, you must kill a close friend, your best friend! Whew, its really a hard ordeal though. He said on one episode that he want Sasuke to become stronger, much like him if possible so he can test his maximum strength. I agree that Sasuke has a big potential of releasing the Magekyu Sharingan bacause Itachi said so, Itachi is bright and strong. He can tell the strength of a ninja because he became the leader of ANBU in early age and I think he based that statement by his experience. Itachi is very strong because Orochimaru needs the body of Sasuke just to be able to have a chance of defeating Itachi, that gave me the idea thats why Orochimaru is in badly need of Sasuke's power. Maybe Orochimaru and Itachi have fought and Itachi was far better than Orochimaru. I guess, Sasuke and Itachi will have their fight. Its really hard to tell who will be victorious and I just hope no one will die before they have their reconcilation.

If I remember correctly, he said he did it to test his powers and said that they deserved to die because they were corrupt or something. He also killed his best friend who's name escapes me to gain Mangekyou Sharingan. He let Sasuke stay alive because he wants him to get stronger through pure hatred of his brother for ruining his life.

I think that is one way of testing his new acquired sharingan. And probably this is also one of the requirements for him tio join akatsuki. what u think/