He didn't. He was pretending to be after sasuke's eye so that it will seem like to sasuke that itachi really became evil and will not regret that he killed him when itchi really did everything, even pretending to get sasuke's eye, just to protect sasuke.
He Did It To Force Sasuke To Kill Him, & So he Wouldn't have to kill Sasuke Himself, his teammates Would try & Kill Sasuke for him, that way it wouldn't be so much trouble for Itachi, i mean he already had 2 kill his whole clan, i think he wouldn't want to kill his brother.
