Sins Not Soon Erased
War was a horrible thing; the worst possible outcome of a situation. Carnage, sorrow, thirst for blood, grief, Itachi hated it all. He had seen the evil effects of war when he was very small. To help ensure it never reoccurred, he became a shinobi. As his skills grew, Itachi was recruited to spy on his clan. Since no one was dying from this action, he was willing to perform it.
Then the rulers of the village told him to kill his clan. The carnage from that would be almost unimaginable. But if the Uchiha lived, they would carry out their coup d'éat against Konoha, and many people would die. Either way, there would be casualties. Itachi was trapped by his own beliefs, unable to choose the path of peace. The closest choice was the path of least blood. To him, that path was soaked in his clan's blood.
Now that he had taken that path, regret was his only companion. His sin of war was one that he could only erase with his life. His pacifist nature cringed at what he had done and Itachi wished he could erase it all.