so we all know that both itachi and shisui have the mangekyo sharingan. if itachi killed shisui to get it, who did shisui get it from?and if you have to kill your best friend to get it, and itachi was his best friend, how did he get it?
1) You dont need to "kill" anyone to get MS. You just need to suffer the loss of someone extremely close to you.
2) We dont know that Itachi actually killed Shisui to get his MS. He obviously watched him die though.
3) Shisui may have just watched someone extremely close to him die, like Sasuke did. Sasuke never killed Itachi but still received the MS after he watched him die in front of him.
4) Shisui was older than Itachi. Shisui may have been Itachi's best friend as his mentor, but we dont know who was close to Shisui at this period. Anyone could have activated his MS, we know no one else in the clan and now they are all dead. FYI
-gHost Senju, K.U.S.H. Sage