You do not gain the MS from killing or being near your best friend's death, you gain it after experiencing a tumultuous amount of emotional pain that pretty much psychologically ****s you up to the point where you awaken it, USUALLY the only thing that can cause such a pain is the death of someone very close to you, ( i.e,.. Best friend, Brother, ect,.. ) Kakashi lost all his friends early in his life after he got the Sharingan, I'm betting he just didn't realize he had unlocked it after he encountered Itachi with the MS.