When Itachi as an ANBU, he's got a secret mission from his boss, Danzo Shimura. The secret mission is to slay all Uchiha Clan. Danzo want to slay Uchiha Clan because Uchiha Clan was planning to rebel against Konoha. Beacuse Itachi Uchiha is a person who loyal to the Konoha, he carry out his dutties to kill all Uchiha Clan.
In one night he kill all Uchiha Clan include his family who is known as the stronger clan in the Konoha . But Itachi leaving one Uchiha alive, he is Uchiha Sasuke, younger brother of Itachi Sasuke. Before he left the village, Itachi told the Third Hokage Hiruzen Sarutobi to keep Sasuke keep alive and did not tell about his mission to Sasuke. Because Sasuke thought Itachi killed his family to increase its Sharingan, Sasuke revenge on Itachi and became determined to kill him.
After Itachi left Konoha, he known as an S-class missing nin and Itachi become Akatsuki member. In Akatsuki he partnered with Hoshigaki Kisame, a member of the Seven Ninja Swordsmen of the Mist. After Sasuke killed Orochimaru, Sasuke meet again with Itachi and fight for the last. In the fight, Itachi actually let himself be killed by Sasuke and before he died Itachi gave some power to Sasuke. During his life, Itachi ever met with Naruto in someplace, and that moment he also seemed gave some power to Naruto.
Itachi Uchiha is a person who really loyal to his village but he more loved his brother Uchiha Sasuke than Konoha Village. He deserves to be called as a Hero.
Itachi Uchina has Mangekyo Sharingan, with his left eye he can use Tsukoyomi, a genjutsu that traps an opponents mind into illusory world. With his right eye he can use Amaterasu, a black flame that can burn anything. And his ultimate techniques is to summon Susanoo the Raging God of the Battle.
Source : Narutopedia
