Sasuke,
I pray you never forgive me. My time as a Konoha shinobi has served me as both a light and a darkness floating away in the vestiges of my tattered past, bringing me both crippling nightmares and the happiest memories of the best moments of my entire existence on this plane. I do not regret what I have done only because my actions saved you from what would have been a greater tragedy. I do, however, humbly request you take a minute and read on to the rest of this letter and know that every letter is written in complete and honest truth.
It has been seven years since that fateful night; seven years that I have dutifully kept my secret hidden from querying minds. It is only now I break the barrier surrounding my greatest shame to you; my foolish but infinitely precious little brother. I understand the fact my actions have left you confused and alone with nothing to turn to but revenge. I regret things couldn't have had a different, more preferable outcome; but like all trained shinobi, we both know that life does not always bend to our will, nor does it ever answer our prayers. Life merely does as it pleases- a predetermined force, regardless of those who wish to change its course.
Since the time of my early childhood I was regarded as the pride of the Uchiha. This will not come as a surprise to you seeing as how Father so foolishly overlooked your promising abilities, something that has always grated on my nerves greatly. My skills were exemplary, even by our clan's standards- a prodigy, a genius, they said. However, it meant nothing to me. The only thing I wanted- what I still want- most was to protect the village at any cost, and ultimately, to protect you at any cost. You most likely recall in your childhood that I was more of a quiet, gentled shinobi, one who was against unnecessary violence and bloodshed. This is due to my involvement in the Third Shinobi War when I was a very small child. The effects were greatly traumatizing- I still have many nightmares about the unforgivable acts that I committed in the name of Konoha, and it turned me into a pacifist. I wish for peace for everyone now- especially for you, Sasuke.
Father pushed me rigorously all throughout my childhood, so much so that by the age of thirteen I was leading ANBU squads consisting of full-grown adults into battle. It was around this time I caught wind of a clan discussion led by none other than Father, who wished to overthrow Konoha and take on his 'rightful' place as Hokage. I was required to spy on the higher-ups of Konoha and report their actions to Father. I did so at first, but the thoughts of the impending coup d'etat went directly against my pacifistic ways and would ultimately destroy any vestiges of peace the village held. And so, I reported our clan's intent to the Hokage himself and began passing on information to he and the Elders, sufficiently making myself a double agent. We tried everything- the Hokage attempted to reason with the Uchiha on multiple occasions, anything we could, but none of it proved to have any effect.
Everything came to its climax when I was burdened with the order to exterminate the entirety of the Uchiha clan, ordered to leave no one alive. This came not from the Hokage who was against it, but the Elders who overrode his desire for a bloodless end to the hostilities between the Uchiha and the rest of Konoha and tasked me with the massacre regardless. Your older cousin Shisui died shortly before the massacre and presented my with his left eye and told me to use it if it would save the village from destruction. His right eye had previously been stolen by Danzo. He then asked me to kill him so the rest of the clan wouldn't find out about our exchange. I complied, and his body was found floating in the Naka River.
I happened upon the existence of Uchiha Madara in the time before the attack and made a deal with him- if he did not harm Konoha, in exchange I would allow him to aid me in the massacre as vengeance for when they turned their backs on him as the previous clan leader in favour of a Hokage who was not of our blood. He agreed. It made me ill to request such a thing from the likes of filth, but it had to be done in order for all this horror to come to an end. That night we killed every member of our clan but one. Even when killing Kaa-san and Oto-san, they still claimed they were proud of my skills as a shinobi and both loved me. Driving the blade of my sword through their hearts killed me inside. To this day my guilt crushes me with its intensity.
I lied to you in order to keep you safe. You believed I was the villain as I hoped; I wanted you to despise me. I ordered you to one day surpass and battle me both so you would become a strong, honourable shinobi and set me free of my misery and regrets the moment you killed me. That night I had also caused you to awaken your Sharingan- under terrible circumstances but nonetheless, another step towards becoming a formidable foe.
I will always love you dearly. As my little brother you are by far the most important person to me, and in death I will always watch out for you and urge you onwards. I have equipped you in every way I could think of. I apologize profoundly that I could not keep our family from being slain. Please trust me with this one thing, if nothing else- if there was any possible way to save both our family and Konoha, no matter how dim the chances were to succeed, I would have taken that opportunity and used it to the very best of my ability. I only ever want the best for you, Sasuke, and I promised you on the night the Kyuubi attacked Konoha that as your big brother, I would stop at nothing to protect you. I hold myself to those words today, as I watch you in the afterlife. My biggest regret at this time of my existence is we were both robbed of the ability to know each other properly and to hold a bond that only brothers could have. I sincerely wish we could have lived in harmony and happiness side by side, in Konoha and surrounded by our family, friends and teachers. That is all I have ever dreamed for. But, little brother, perhaps the afterlife will not be so cruel to us and we will finally be able to have the life we always wished for.
I am deeply sorry.
-Itachi, forever your big brother.