why itachi killed his clan?

• After the incident of the Nine Tailed Demon Fox attack at the hands of Obito Uchiha, the top leaders of Konoha suspected that the Uchiha Clan was behind the attack, because the Sharingan has the ability to control a Tailed Beast.

• due to this attack they decided to watch over the clan

• which caused Obito to declare that they were forced to live in a small corner of Konoha, which would become the residence of the Uchiha. When Clan members began planning a coup d'état led by Fugaku Uchiha, they placed Itachi Uchiha as a spy within the ANBU to learn about Konoha's plans.
• but nevertheless they did not expect that due to the peaceful nature of Itachi, he would become a double agent, giving the main authorities of the village information about the Uchiha's plans.
• This he did because he knew the havoc they would cause inside the village and that the destruction it would cause was going to allow the infiltration of enemy villages creating a conflict that would bring another Shinobi World War. As the Uchiha attack approached, Hiruzen tried to find a way to negotiate with them, but time was up and the advisors of the Third Hokage sent Itachi to kill the Clan.