Why did Itachi kill his clan? Story behind the infamous Uchiha Massacre

Itachi Uchiha is one of the most loved characters from the Naruto Universe. And we can say that there have been a lot of questions related to the things he has done - some answered previously and some lost in oblivion.
One such question that always makes rounds of the internet is why Itachi killed his clan. Now, there are multiple answers available for the same, some official and some unofficial. However, the only thing we would like to say here is that Itachi was loyal to Konoha village - loyal enough to go against everything and be remembered as a villain for eternity.

Without beating the bush, Itachi killed the Uchiha clan to stop the rebellion. For those who are unaware of the history, The Uchiha Clan was unhappy with the fact that the elders in Konoha were treating them as outsiders.
And after the attack of nine-tails, the clan was constantly being doubted. Due to this, Itachi, who was then working in the Anbu Cops, was asked by Danzo to keep eradicating each and every member of the Uchiha clan except his brother. This was the only way to save Konoha village from an all-out war and keep the life of Sasuke safe from any Hurdles.
Despite knowing how difficult it is going to be, Itachi kills each and every one of the clan, only leaving his brother behind. Now, he never wanted to become a hero, so he simply went to join Akatsuki, an organization consisting of rogue Ninjas from different villages. But here, Itachi also had a single motive - to keep a check on Akatsuki and make sure that it would not harm Konoha.
Later in the events of Naruto Shippuden when Sasuke finally learns about the sacrifices of his brother, he is adamant about taking revenge from the culprits. And the most satisfying part of the whole saga was Sasuke brutally killing Danzo who was the mastermind behind the Uchiha massacre.