They planed a revolution and being a pacifist, Itachi feared the war that would come.
He was also ultimately ordered to do so by the politicians within Konoha.
Itachi had an undying love and devotion towards Konoha (evident by him joining ANBU). However, he realised that if the Uchiha revolution were to continue, it would cause a massive war that would gravely weaken Konohagakure entirely, thus making them a target for all the other nations to attack and conquer, hence causing relentless bloodshed and Konoha's demise along with everyone Itachi held dear to. So, he destroyed the clan by himself to save Konoha and also because of the deal to salvage Sasuke as well, for he wanted more than anything for Sasuke to live.