the uchiha were planing a coup d'état on the leaf that is what madara predicted but the three elders planted itachi as a spy to keep the info on them Then itachi was received the mission to kill the entire uchiha clan. itachi kept sasuke alive not to measure his vessel it was because he just couldn't bring himself to kill his little brother.
Then he went to the third hokage he told him if you lay one hand on sasuke ill kill you instantly then he left without a word. He was to be considered a traitor due to the assassination. Itachi wanted sasuke to kill him so he could be known to revive his clan but itachi didnt expect sasuke to join orochimaru which his plans were failed but sasuke still kills him then finds out the truth by madara
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He was instructed by the third hokage and his advisors because they were planning a revolt against the leaf.
The Konoha Council and Danzo were the ones who told Itachi to kill the Uchiha clan.
Itachi Uchiha part of the Uchiha clan also a member of the akatsuki
In short: born into the Uchiha clan, became friends with Itachi Uchiha, gained the Sharingan and Mangekyo Sharingan, was assign to spy on Itachi by the clan when they where planing a coup and Itachi was acting strange, tried to stop the coup and got one of his eyes stolen by Danzo, gave his other eye to Itachi and killed himself.
This might be a spoiler, but Sasuke kills Danzo because him and the other elders around the third hokages. People ordered itachi to kill the uchiha clan. Itachi was only staying loyal to the leaf village, but he left Sasuke alive and told the hidden leaf to keep him safe.
After Itachi massacred the clan, Sasuke's innocence was corrupted, his ideals and personality changed severely, becoming cold, lonely, cruel, cynical, somewhat arrogant, and with the next nine years of his life devoted to killing Itachi. Although now Sasuke REGRETS killing itachi after remebering what a good brother he was.