Itachi has no scars from battle.
This is not surprising. He is a prodigy like no other. In a fight, he moves with an almost baffling grace. Elegant black lines of hair, fine red eyes. Try to outwit him—but ah, you can't. He's always so many steps ahead of you. Throw what you want at him and he'll deflect it with ease.
And yet. Lift up his shirt and there are slender pink lines above his navel. Several long, horizontal ones, and innumerable smaller ones. Unclothe him and spread his legs; the insides of his thighs have even more.
Sometimes he feels so sick, all he wants to do is plunge a knife into his stomach. (Is it the sickness in his body or in his mind? It doesn't matter. The Uchiha flesh is heir to many deficiencies. Ninja are often prone to mental illness, but the Sharingan, because it is fueled by strong emotion, exacerbates emotional instability. Generations of inbreeding to purify their wretched eyes have taken a toll as well. Look no further than his father's abnormal jaw, or even the lines on his own face. The entire clan was diseased and rotten. He knew of too many Uchiha children born with weak hearts and lungs, who died soon after birth. It seemed, at his own birth, that he was one of the lucky ones, but now he knows better. The only healthy — Itachi cannot call him lucky — Uchiha child born to their clan in recent years was Sasuke.)
But he can't. His body is a tool in service to Konoha, even after all this time, and he still has a task to do. And even if he were to lose faith in Konoha, his body is, without a doubt, Sasuke's to kill. To deprive his little brother of that would be unfair.
This is the compromise he reaches with himself:
He carries a bag of salt with him. When he can't stop thinking about suicide, he leaves Kisame alone for a few hours and finds a dark room for himself. He cuts a shallow line in his skin with a kunai, pinches some salt out of the bag, and sprinkles it over the wound. The amount of blood is minimal, but the pain is not. It burns and itches him for at least a few days afterwards. It's not very satisfactory, but it will make do until Sasuke is ready to kill him.
That's all he thinks about as he lies in the dark, breathing, a thin rivulet of blood making its way across his skin. He imagines Sasuke standing over his body. Sasuke shoves a sword into his heart. It will be a moment of perfect symmetry, he thinks. At that moment, the misery in both of their hearts will unravel. Sasuke will have avenged his clan and can return to his village as a hero, and Itachi will cease to feel so much guilt.