please correct me if im wrong but can we really be sure that itachi did kill uchiha shisui? yes, itachi did have almost complete mastery of the sharingan but someone of shisui's level wouldnt have gone down without a fight and yet there wasn't even a mark on itachi. Also everyone assumes that to obtain the mangekyou sharingan you need to kill your best friend, if that is the case then why did madara and his brother achieve it through work alone? it makes more sense to assume that you obtain it from a mental change or mental anguish in which case itachi would only have to have witnessed his best friends death.