Disclaimer: Naruto belongs to Masashi Kishimoto. There's no way I'm making money from this and absolutely no way I'm affiliated with the creative process of this series.
He Forgot the Kunai
Sasuke is afraid of the dark. At seven, he should be over it by now, but there's something about not knowing, not being able to see that makes his insides tremble and turn cold. Ninjas shouldn't be afraid of the dark—it's their friend, their lover, their protection during missions.
Sasuke wishes for the dark now. Wishes with all his might that the full moon would wink out of existence so he can't see anymore. If he can't see it, it's not there, it's not real. Stupid reasoning.
Mother and Father are lying on the floor, blood pooled underneath them. It's stopped spreading, so their hearts must have stopped beating a long time ago. Itachi stands there, his unreadable face ignoring the blood and the bodies, eyes only on Sasuke. How long has Itachi stood there, simply waiting, watching as the blood spread from the bodies of his parents?
The moonlight makes the blood look so strange, a kind of reddish quicksilver that almost seems alive by itself. It reflects the moonlight quite well, really. He wishes for darkness again.
Itachi's words register in Sasuke's mind but they don't make any sense. To test his strength?
And suddenly Sasuke knows how to fix it all. He knows how to get the darkness to come. With a yell he charges straight at Itachi, making it seem for all the world that he is trying his best to kill his older brother.
Itachi is the genius of the Uchiha Clan. He graduated at seven, achieved the Sharingan at eight, became a Chuunin at ten, an ANBU squad leader at thirteen.
So when Itachi moves and Sasuke finds a fist implanted in his gut instead of a kunai, the younger boy is much more surprised than hurt. After all, ninjas aren't supposed to leave a functional enemy behind.
Sasuke is on the floor, trying to regain breath. The moonlight from the window is in his eyes, and his parents' blood is within his field of vision. Instead of a fatal wound, Sasuke is just winded.
This night has changed Sasuke. From this point on he'll shun the light and pray for darkness. He'll swear revenge, forsake everything because of this night. He'll drag Itachi down into the dark with him.
Sometimes even a genius can make a fatal error. Itachi's was simple, a screw-up of the basics:
He forgot the kunai.
Sasuke will make sure that he doesn't do the same.