Prompt: driven to insanity
Itachi is dying.
That's not important, though; there are medicines for that. The more immediate problem is that he's also going blind, and there's nothing that can stop it.
He's tried, naturally - healers and ancient scrolls, obscure specialists in diseases of the eye. Most of them don't even know what the Mangekyo is, let alone its side-effects.
He contents himself with the fact that he won't need the eyes for much longer. He's planned this, started planning it nearly fifteen years ago when he first met Madara. He knows: Sasuke is important, Itachi is not. Sasuke must live, and Itachi must therefore die. He thinks of it, his death, and perversely this is what makes him keep living. To die in the right way, to die with as much honour as he is permitted now. To die at the hands of his brother, who will take his eyes knowing nothing of the sacrifices that Itachi has made to keep him alive.
The thought is bitterly satisfying. Dulce et decorum est, he thinks, without irony.
Kisame thinks he's crazy.
Itachi thinks this is rich, frankly, but Kisame admits his madness so allegations of hypocrisy are met with a snort. Itachi acknowledges his own madness, in his own way: he knows that normal people don't plan like he does, don't see the connection between allying himself with a traitor and holding down his best friend's head in the Nakano river until he stopped moving and protecting his little brother because eight years ago they looked into each other's eyes and he couldn't bear to kill him. He failed his village, but Sasuke lived and lives and will live, and for that anything is bearable.
Shisui was his only friend.
Not the person who was closest to him - even now, that would be Sasuke - but an actual friend. Some might say best friend, but that implies that Itachi had other friends for comparison, which is not true. He wasn't friendly with his peers, he had little thought for his clan, and his relationship with Sasuke could never be expressed in so simple a word as 'friend'.
He liked Shisui - likes, even, because death does not end affection. In Shisui, Itachi found something that approached understanding. He was intelligent and talented - gifted, even. He was quick to anger, but also quick to laugh. He was well-regarded and popular, but for some unfathomable reason, at age seven he proceeded to attach himself to the side of his four-year-old cousin Itachi. Even Itachi doesn't know why - he never asked.
Shisui might be who Madara means when he says that Itachi had a lover. Itachi wouldn't have used the term and he doubts that Shisui would have either, but Madara, he has realised, is something of a romantic. In truth, Itachi simply liked Shisui enough to allow his gentle kisses on his mouth and neck, and to let him to cup the curve of Itachi's skull in his broad-palmed hands. He liked the way Shisui's fingers curved around his neck, how he was the only person who could make Itachi laugh.
None of this keeps Itachi from murdering him.
Madara is a necessary evil; something to be borne for the good of the village, of the clan and ultimately of Sasuke.
Itachi is trained in the true ninja arts by this embittered ancient warrior: how to lie to one's peers, how to betray one's friends, how to kill one's fanily, all without a shred of remorse.
Sasuke was, is and forever will be the most important person in his life.
Everything he has done, does and ever will do is for Sasuke. Even when his limbs become weak and he coughs up blood, the strength of his purpose burns in his eyes like the flames of Amaterasu. A long time ago, there was more to him - a village, a family, a friend. Now, all that has been burnt away to leave a shell intent on one thing only: the survival, protection and elevation of Sasuke. This is his legacy.
Itachi does not close his eyes, but he does go gentle into that dark night.