Why did Sasuke change?

Answer 1

Well I'm not sure if you know about the clan massacre or not but that was a big influence on him.

POTENTIAL SPOILER:

Itachi killed the entire Uchiha clan and lied to Sasuke about his motives. He led Sasuke to believe that he was a cold hearted killer who pretended to be a sweet, loving brother. In reality, Itachi always was the sweet loving brother he said he wasn't. He killed the clan in order to prevent a coup de'tat which would've potentially caused another war. Itachi had seen war before and didn't want that to happen to the country he loved, plus he didn't want his brother to have to experience that at such a young age. So, killing his heart, Itachi slaughtered his own family as a mission for Konoha. Sasuke ended up killing his brother and only AFTER killing him did Madara (another Uchiha and the main villain so far in the Naruto series) tell him about Itachi's innocence. Sasuke, having realized he just killed his loving big brother in cold blood, is overcome with emotion and passes out. From then on he's a changed person, he's confused and doesn't know who to blame. Everyone in his path is in his quest for revenge. He's confused and in my opinion needs therapy. That about sums up his reasons. His entire world was turned upside down and now he just wants something to fill the emptiness and the ache. He wants the world to pay for his misery. Lovely, no?

Answer 2

bicose the chunin exam start and kakashi givet to him