He'd run in fear, knowing he was no match for his brother, and at the last moment when it looked as if he were going to die, his Sharingan had activated. When his Sharingan had activated, he'd turned around and fought. It hadn't been enough however. He'd been easily defeated and That Man had fled. Strangely enough, there had been tears in his eyes as he did so. Why had he been crying though? There was nothing the sort of person who would kill his own family just to see how strong he was could be sorry for.
That Man had told him before he fled the night he'd killed the clan that in order to defeat him he should run and cling to life hating him all the while, and not come back until he had eyes like his, the Mangekyo Sharingan. In order to get the Mangekyo Sharingan, he would have to kill someone close to him, someone he cared for, someone who cared for and trusted him. He would have to see the betrayal in that person's eyes as he or she fell to the ground dead the way the members of his clan had. He would have to become like That Man.
Was Avenging the clan worth becoming another Itachi? A year ago, he would have unhesitatingly said yes. Heck, six months ago even. Hell, even a month ago. But now, as he stared at the betrayed faces of the members of his clan, he found himself hesitant. He wasn't sure if he should be hesitant though. If he survived today and got another chance, he would do it for them. He would become just like That Man for them, but on a much smaller scale.
Then what? Where would he go from there? Those who slay their comrades were on almost the same level as those who killed family. If he went through with it and killed someone to attain the Mangekyo Sharingan, he would never again be welcome in Konoha. He needed to be in Konoha to restart his clan though. He needed to be in Konoha to fill the compound with his descendants. He wouldn't be able to do that if he killed another of Konoha's citizens, especially if he did it for something like the Mangekyo Sharingan.
"Little brother, are you not watching?"
"I don't think you are."
"I can fix that."
Suddenly he was strapped to something so he couldn't flee even though there was nowhere he could run to in this place, and his eyes were forced open. The killing started again. And again. And again. And again...
Eventually, after what seemed like forever, the world finally went dark.