Originally Posted by cheese4u
I know this seems far fetched but I think Itachi may have been lying about that, or at the very least not telling the whole truth. I think Itachi just told Sasuke that so he won't get attached to his emotions, so he can be more motivated. Think about it Itachi claimed he killed Shishui to obtain the mangekyo sharingan, but we never saw Shishui at all during the show, or his deceased body. It could be Kishi hinting that Itachi didn't do it, and maybe MS is gained in another way.
I agree, it doesn't sound too crazy that Itachi might just make that up to make Sasuke consider darker paths in order to achieve power.
This can then all link up to why he killed all the Uchiha in the first place, maybe he saw them as too-friendly, too-soft and wanted to rebuild the clan as a more evil, more powerful organization similar to Akatsuki.