Sasuke didn't really want to fight his brother,deep down. He knew that if he gave up on trying to fight Itachi,he would lose the resolve to become strong, and he wouldn't have a reason to be a ninja-he wouldn't have a nindo.
After massacring the clan,Itachi had no hope in becoming a good guy again. He was a wanted criminal and a runaway. He joined the Akatsuki so that Sasuke could find him.
