if you look here sasuke talking with hes father about new types of sharingan , why would he tell him about MS if you need to kill someone ...
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and itachi had MS after he found out that hes friend is dead... which could be that MS is emotional condition of losing someone really close to you, you dont need to kill him but just to lose him... itachi was saying sasuke to kill hes best friend to get MS cuz he knew that sasuke is strong in emotions, so he needed to do something to brake hes emotions so that sasuke can kill hes best friend ...
just remember he didnt kill naruto....