Itachi Kills his Clan
Itachi met Obito Uchiha, and slaughtered the entire clan on that fateful day. He even had to kill the love of his life Izumi Uchiha, but before that he did put her through a genjutsu, where she dreamed of all the good things that would’ve happened, had the coup d’tat not taken place.
Finally to sign things of, Itachi had to kill his parents.
Fugaku Uchiha, Itachi’s father asked Itachi of a favor. He asked him to protect Sasuke till the end, hence Itachi joined the Akatsuki from where he can keep a close eye on Sasuke as well.
After the third hokage died, Itachi returned to Konoha, to tell Danzo that he’s still alive and if he did something nasty such as try to steal the Sharingan, then Itachi would leak all the secrets of the village to the Akatsuki.
Itachi experienced war at a very young, he knew what it was to lose people close to you, he knew the thirst for blood and the emotions that flowed along with it.
Hence he decided to take the burden on himself and bring down the Uchiha. So, on one fateful night he drew his sword, comprised with his inner self and ripped the clan apart, leaving only his younger brother Sasuke.