1) He let his emotions get the best of him. (When he attacked Itachi with Chidori)
2) He abandoned his friends and betrayed his village. (When he ran to Orochimaru).
Seriously, what was Sasuke thinking when he tried to attack Itachi? He must know that Itachi is stronger than Kakashi, and I doubt he really believes he could have beaten even Kakashi at his current level.
He let his emotions get to him, and all he did was get himself hurt.
If I were Sasuke, I would do things calmly, wait until I was at least a Jounin, and even then I would use tactics and tricks to try to kill Itachi. I would also use my friends to help, especially Nara Shikamaru.
But instead, Sasuke did a frontal assult on Itachi (while Kisame was nearby) when he was just a Gennin. Thats just stupid.