Originally Posted by noyoyo
The basic way to get the MS is to have someone close to you die.
Sasuke because of Itachi's death, Kakashi because of Obito's death (although unconfirmed since he had it for a long time), and Itachi got his because of Shisui's death.
It doesn't necessarily have to be you killing your best friend, it just has to be someone that's really important to you dying (perhaps in front of your eyes).
If Sasuke kills his best friend (as Itachi said), it'd fulfill the above conditions and he'd obtain MS. Itachi just gave him a way to get it, which fulfills the conditions without actually telling him the conditions.
There are possibly other ways to get it, although unlikely.