Naruto: The Real Reason Itachi Killed The Uchiha Clan
Itachi Uchiha is one of the most compelling characters in all of Naruto lore. The prominent and powerful son of a noble and respected clan, he murders almost his entire family in a massacre that sets in motion many of the series' driving plots (at least as far as younger brother Sasuke is concerned). This central event is used by the series writers to explore plenty of compelling concepts. What does it mean to be loyal and patriotic? How can a desire for revenge corrupt even the most promising young people? And to what lengths will a government go to maintain power?
Of course, to understand any of these questions or the way that the series grapples with them, you have to first wrap your head around why Itachi committed this nearly unspeakable atrocity. The short answer is that Itachi Uchiha killed his family to protect his homeland. The long answer (and in the sprawling world of Naruto's 700 some-odd episodes, they're all long answers) is a bit more complex.
Itachi Uchiha killed his clan to prevent a coup
The Uchiha clan were among the founding families of Konohagakure. They formed the world's first Shinobi village during the feudal period in the Naruto world, along with their rivals in the Senju clan. Other clans and fiefdoms followed their example, pulling the world from the small-scale fighting of the "Warring States" period to the global superpowers that exist at Naruto's start.
Though they were able to put aside their differences to end the bloodshed and found Konoha, the lingering resentment between the two powerful families never fully evaporated. Ill will grew as the Uchiha clan were frequently passed over when the time came to assign roles in government of the village. Senju family members held power from the very first days of the village and appointments for their allies and clansmen followed. The Uchiha did not help protect the village from an attack by a great beast, which led other clans and village people to be ever more suspicious of them. The clan was steadily removed from the political affairs of Konoha and retreated to their own corner of the village, where tempers flared and clan leaders stewed in their own indignation.
Clan leader Fugaku Uchiha decided to switch things up, plotting a coup against the leadership of the village. When word got around to village elders, they presented the clan's strongest young member with a nearly impossible choice: wipe out his own clan for the good of Konoha, or die with them.
Itachi Uchiha was trying to protect his younger brother
Itachi Uchiha agreed to carry out the massacre with some caveats. Danzo Shimura knew that Itachi was wildly protective of his younger brother Sasuke, and used that information to sweeten the deal. He said that they would leave Sasuke alive if Itachi would carry out the attack and take the fall. He agreed, not knowing that his decision would send Sasuke on a quest for revenge, culminating in Itachi's death.
As proven by his actions, Itachi was deeply loyal to Konoha, and took his oath as a village shinobi very seriously. On top of the idea that his younger brother might die, agents of Konoha told Itachi that another world war could break out if his family was successful. Itachi wished to avoid this at any cost, and carried out the awful plan.
After the massacre, his status as a murderer and traitor to his clan allowed the still secretly loyal Itachi to gain access to the splinter group Akatsuki. He continued to serve as an agent of Konoha, feeding information to his village on the terrorist cell's plotting until his death at the hands of his younger brother.