I subscribe to the belief that Itachi told Sasuke a lie. "I did this to test my abilities." It's this sort of thing that would rile Sasuke. And, as we know, Mangekyou Sharingan is merely an illusion. A powerful illusion, yes, but Itachi can construct the world as he sees fit.

Ever since I knew about Itachi, my own hypothesis was that there is another force at work, and in order to keep this force from obtaining the power of the Uchiha clan, Itachi kills everyone except himself and Sasuke, for he feels they are the only two who have any chance of fighting against this force. Itachi says what he says to Sasuke in order to motivate Sasuke to become stronger, as he himself believes that Sasuke can surpass him.

Itachi may seem cold, but it's obvious he still has some attachments to Sasuke. Check out the finger twitch in episode 81. For someone who's so calm and able to move with such precision, a twitch like that is pretty significant.