It was a small, meaningless moment between them that triggered it. A mere passing, with Sasuke sitting atop his brother's shoulders, staring down at the very alone blonde boy in the middle of the street. In normal circumstances, it could be interpreted as pity, because that is usually the look that comes when one has something someone else obviously doesn't. However Sasuke understands loneliness, and he had been alienated by his attempts to keep up with his brother, to win his father's pride, to be something other than the little brother of the man who could kill relentlessly and efficiently without hesitation. So the look that passed between Naruto and himself was not pity, but understanding, sympathy. However, the damage had been done, and Naruto, who was at best hasty in his conclusions, was set to prove the aloof raven haired boy wrong.
As time passed the boys grew up and taunted one another and worked so as to handicap the others abilities. Sasuke had found Naruto's cold treatment unsatisfactory and had rejected it with a zealous anger, and Naruto wanted to be greater than he was, wanted to be Hokage, and the Hokage did not lose battles to snobbish and arrogant men, no matter how exclusive their ninja techniques. As such, the boys grew up abandoning each other and using every resource surrounding them for their blood lust to be fulfilled.
Very quickly, it went wrong.
It started for Sasuke as he watched a long limbed lanky teenager walk down the grovel path behind a gathering of women. He was not particularly beautiful, or developed, but he was so like Naruto, that it hurt to watch him. He had Naruto's habit of gritting his teeth at annoyances, and severe lack of control over his temper. His eating habits consisting of ramen and canned soda was apparent also. Common traits in teenagers perhaps, but that did not disturb Sasuke. He wanted him. Not for sparring or romance, but he wanted him to fill a very alone and very dark place within him, which had once been illuminated by the Kyuubi container. He wanted to snag the boy, drag him into the cover of the nearby bushes, and then have his way with him, as he ravished him again and again.
Sasuke took one step and then another, and then the next until he was far, far away from the Naruto-like boy.