Ok here's my theory. I believe that itachi destroyed the uchiha clan because he thought they were weak. however the reason why he left sasuke still alive was so that sasuke can start the uchiha clan over. but wait, i think that itachi left one more uchiha alive, i believe that itachi left a girl uchiha alive and took her with him, or she ran away because of fear. but i think that latter on in the stroy we are gonna see another uchiha come along and sasuke is gonna find her and then they are gonna rebuild the uchiha clan.