Maybe it's old but since Uchiha kills their best friends to obtain the MS and brothers, parents etc for ems why Itachi don't had it already? Furthermore he didn't kill Shisui but he had the MS before he slaughtered his clan? Plot hole?
No, they were just ANBUs, not in root.itachi, shisui were in root so they were pitted against their friends by danzo, just like sai and his brother, so they both awakened MS, later they became friends and danzo stole shisui eye........