Disclaimer: I don't own Naruto.

There is not usually a defined moment in a sensei's career training genin that can be positively identified as his greatest failure.

But then, Kakashi isn't like most ninja.

There are two moments in his career that have been described as the irredeemable black marks on his soul.

Kakashi had a team once. His team was made up of a cold little prodigy bent on revenge even at the cost of his soul (a broken boy who bore his hurts on his shoulders), a little girl who whined and couldn't pull her own weight (a girl who wanted nothing more than to be accepted by the cold boy of her dreams), and a loud, obnoxious little brat who was better than anyone and hurt more than anyone ever guessed (His hurts were borne where the world couldn't see them; he didn't advertise them the way the prodigy did).

They were like puzzle pieces that almost fit, but didn't quite mesh. Over time, it almost seemed like they would survive as a team, that they had a future together after all. The prodigy got warmer, the girl started to grow up, and the brat began to know what it was to be loved.

Then, everything just fell apart. The boys fought, the girl screamed, and Kakashi only watched. He only watched, as the world of Team Seven imploded and collapsed into ash-laden debris.

But that wasn't the worst of it.

Team Seven was a team that was never meant to be; the puzzle pieces crashed and fell away from each other. Naruto, Sasuke and Sakura would have been better off if they had simply never met, if they had never met to hurt each other and kill each other in small, devastating ways.

They fell apart, just as they were always meant to. It was hardly a failure when they were never really moving in the same world to begin with.

But the world moves on. And that's where Kakashi's greatest sin grows like a rampant parasitic plant, its growth unchecked.

Sasuke doomed and damned himself, but Naruto and Sakura's lives were still salvageable. There was still some light in their growing darkness, and it could have been saved.

They should have been made to forget Sasuke, or at least to resign themselves to his departure. They should have been dragged out of the darkness into the light, away from their fear and their grief, and Kakashi was the only one who could have done it.

But instead, they were left to stew in darkness. They were left to their grief, to their pain, to their memories, and it consumed them. In their minds and hearts, they stayed the children that had so wanted Sasuke to come home, long after their unformed souls were dead and something new and dark had emerged in their places.

Kakashi's greatest sin was the death of Naruto and Sakura's hope, and that more than just their hope is dying with each passing day, and he does nothing to stop it from ruining them.

His greatest failure was not that he allowed Team Seven to fall apart.

It was that he didn't pick up the pieces later.