Sasuke decided to take revenge on the leaf for what they did to the Uchiha & to Itachi. If you were sasuke and trained your whole life to kill your so-thought evil brother, and then figure out that he actually loved you & the mission to annihilate the uchiha was given to him by the leaf, what would you have done? Would you follow your brothers wishes and return to the village that made his life hell? or would you do what sasuke is doing? Take revenge on the leaf and redeem the uchiha and your brothers sacrifices?
P.S. remember that your in sasuke's shoes and placed in the same situation he was in when told the truth about his brother.
P.S. remember that your in sasuke's shoes and placed in the same situation he was in when told the truth about his brother.