He had beaten him. After all these years, he could now hold his head up high and say he had avenged his family's murder. His brother lay before him, bloodied and near death. He could not go anywhere. All that was left was to kill him and return with his head as proof to his victory.
All Sasuke had to do was simply take a kunai and watch the life leave Itatchi's eyes that now lacked the Sharingan and had returned to their former black. But he couldn't. No matter how much he wanted to see his niisan's head roll, he couldn't. He was Sasuke's older brother, no matter how much he wished otherwise, and he couldn't kill him. He was still family, his only family.
As Sasuke watched his older brother labor for breath, he made a decision. His brother would dictate him no longer. And just like when he decided to not kill Naruto by disobeying his brother's last words too him before he had left the village the first time, he was once again going against his brother's "wisdom". He was NOT going to be left as the last Uchiha, at least not if he could help it.
Having made up his mind, Sasuke did what he wanted; all the while believing he was completely out of his mind. The once avenger put the kunai in his hand back in the case on his thigh and leant down just as his brother loss consciousness because of blood loss. Sighing, knowing he would be carrying dead weight for a while, Sasuke lugged his brother onto his bloodied shoulder. Sighing slightly once more, Sasuke's main thought as he left the destroyed clearing was he hoped Naruto wouldn't get too mad at sharing their apartment with an S-class missing-nin.