well, apparntly, Itachi said all those things just to piss off Sasuke, and he also kills the clan just to get Sasuke mad at him...

if Itachi was able to kill shisui (whatever that sucide guy name is), i don't think that anyone in the Uchiha clan could rival him, so he decided to force Sasuke (his brother, and therefore, the person who is most likely to have the same strentgh as him) to get better and to hate him...

but becuase he was afriad that someone else will attack Sasuke, he contatcs the largest crime orginazation, and promises to work for them as long as they promise not to hurt Sasuke. but Orochimaru hears that, and quits th akatsuki...