Itachi Uchiha is Sasuke Uchiha's elder brother, and many fans see him as one of the best characters in manga and anime. Itachi kills the whole Uchiha clan, sparing only Sasuke, making the latter detest him and wish to kill his older brother, whom he once looked up to, according to Naruto.
Long story short, Itachi assassinated the Uchiha clan on orders from Danzo Shimura and the elders of the hidden leaf village (Konohagakure) to put an end to the clan's plot to take over the village.
Rising tensions between the Uchiha Clan and the Concealed Leaf Hamlet:
From the beginning, when the village hidden in the leaves was founded, many people, including Hashirama's brother Tobirama Senju, dreaded and distrusted the members of the Uchiha clan.
Itachi was placed in the Anbu Black Ops by his father Fugaku, who was masterminding the coup, to operate as a spy for the Uchihas due to his brilliance and outstanding understanding for his age.
To fully comprehend why Itachi made the decision, we must examine the scenario from his point of view. Itachi was scarred as a child by the events of the Third Great Shinobi War, which caused him to despise bloodshed and violence.