I actually agree with this theory because, Itachi could have killed kakashi, but yet he did keep him alive, (very wounded yet alive), and about getting the mangekyou sharingan, If the requirement for it is to kill your best friend, how did kakashi get his? i am pretty sure he didn't kill any of his best friends. I'am not sure if this has already been revealed or anything but if it hasn't, then the requirement for the mangekyou sharingan must be something different, or kakashi found some way around it. but then again, that still raises the question why would itachi tell sasuke that it was the requirement?