Obtaining Mangekyo Sharigan
Ok so Itachi tells sasuke that to obtain the MS you must kill you Best? Friend.
Now think about it, Itachi must have been 17-19ish when he kileld his mate, and clan. Do you really think he went to the uchiha library and looked up and saw Ah... if I kill my friend then, I can be awesome. I doubt that they would have a book, so then could he have heard of it by rumors, but would the clan realyl want the rumor, Kill your mate and you can get a wicked sharigan going round? I doubt it. So mabye, for whatever reason Itachi slayed the clan for, he didnt actually know he was gonna get MS, mabye he got in a fight with his mate, got MS then wondered what would happen if he killed the rest? He may not know why exactly he got it and mabye thats why Kakashi has it alos, ( I know Kakashi has a sligtly different one - and mabye it has different requiremtns ) So itachi just tell sasuke the onyl way he knows, mabye thats not exaclty right.
This post has no relavence to anythign and is just speculation, just wanted to share the idea.
Sorry bout the spelling...