He knew, a lot more than most, about what could drive people towards revenge.
When his father died, Kakashi had had no one to blame for the death apart from his father himself. He could have blamed the villagers, for shunning him, but ultimately it was Hatake Sakumo who had broken the rules. It was Hatake Sakumo who had taken his own life. Instead of hating the people he knew were not really to blame, he put all his energy into being a better ninja. One who would protect the village at all costs. One who would not break the rules, even at the expense of a comrade.
Obito was next. Uchiha Obito, who not hours before had shown him the right thing to do, and finally broken (or at least damaged) the walls Kakashi had built. Walls that made him follow the rules at all costs, abandon his team mates. When he had died, Kakashi had no one to blame but himself. If he hadn't gone off in the first place, it was more than likely that Obito would be alive. His new eye served only to further his guilt. It was proof of the mistake he had made.
After that, it was Rin. Sweet, friendly Rin. He had failed to protect her. His hand had gone right through her chest.
Would he kill all of his comrades?
When Minato had died, giving his life to save the village, a small part of Kakashi had been relieved, even though the rest of him was shocked, horrified. A death he was not responsible for. He hadn't failed all of his comrades - Minato had died a hero. He had left Kakashi alone, but this time there was a solid being to blame for the death.
The nine-tailed fox.
The child it was sealed inside.
Kakashi knew that Naruto wasn't the fox itself, and as one of the ANBU ordered to guard him, he knew what the boy had gone through. But on the anniversary of his sensei's death, when a drunk group of villagers stumbled across the terrified young boy and tried to attack him, Kakashi hadn't intervened. Even though it was his duty, he did nothing to stop them.
He did just the opposite. All it took was a simple henge, and he was just as much a civilian as any of the others. He had joined them and he had hit Naruto - just once - and then he had seen the boy's eyes.
"Please," the blonde jinchuriki had sobbed. "I didn't do anything!" and Kakashi felt sick and had transformed back and rescued the boy, who didn't, couldn't know he was being saved because he was unconscious. The thought that he had let that happen, that he had helped it happen, was too much for Kakashi. His revenge made him feel nothing but sickened at himself. He resigned from ANBU. The guilt still gnawed at him and he allowed it to, using it to help him protect his comrades.
When he had been assigned Naruto along with Sakura and Sasuke for a gennin squad, he hoped and yet desperately didn't want them to fail. Here was his chance to make amends. But training Naruto proved painful. He could see the crying little boy every time he looked at him. On the off chance that he could look past that, he could only see Minato. So he focused on saving Sasuke. Giving him someone to get close to, power to use to protect his friends, was the only way he could see to reconcile himself. He could see himself in the boy, and wanted to save him from making the same mistakes.
As he left the Uchiha sitting on a tree branch, thinking over what Kakashi had told him, he felt relief. He could save his team mates. He could make amends. Maybe he could even forgive himself.
A/N - Argh. This was a lot harder than I thought it would be and I don't think I did a great job, but I've wanted to write this for a while. I keep seeing a load of fanfics where Kakashi saves Naruto, but none where Kakashi hurts him. If anyone wants to use this idea, go ahead (but link me to the fic if you don't mind, because I want to read stuff like this).
Please review and tell me what you thought :D