Itachi tells Sasuke that he needs to kill his best friend in order to awaken his Mangekyo. Why? Itachi knows that to be untrue since he didn’t kill Shisui, so why would he want Sasuke to do that?
Because Itachi is a shit tier strategist when it came to Sasuke tbh. Like the dude ruined his little brother’s mental psyche, and then when he found out that he’s really evil now, insert Pikachu shock meme
I'm late to the party and this is probably going to be a really long post, so bear with me here.
If you look at how Itachi was handled at the beginning of the whole series, you can kind of tell that Kishimoto initially wanted Itachi to be an actual villain and then changed his mind as time went on. I back this up with Itachi's encounters with Kakashi and Sasuke after the Chunnin Exams. These encounters played a key role introducing him as a character (seeing as this is the first time we actually see him and not just have him referenced by another character), cementing Itachi's place in the power scale (by defeating Kakashi with little to no effort with a genjutsu on a scale that has not yet been shown to the reader), and expanding upon the idea that Itachi is the absolute evil that he has been described as by Sasuke up to this point (shown by Itachi subjecting Sasuke to Tsukuyomi after taunting him for not being strong enough).
Now, with those points in mind, we fast forward through the series a bit to the point where we start seeing flashbacks of Sasuke's childhood up to the point of the Massacre. The younger Itachi we see is fairly different to the one the reader knows up to this point, but as time goes on in the flashbacks, the more you start to see the Itachi the reader is familiar with (a murderous psychopath) start to come out. This comes to a head when Itachi is accused of Shisui's murder and he has his monologue about the Uchiha clan. It's right after this event that Itachi reveals his Mangekyou Sharingan. Now the first seeds of “best friend dying equals sharingan power-up” are planted in the reader's mind as they begin to draw conclusions as to Itachi's guilt in the manner. We then again fast forward to the night of the massacre and Itachi telling Sasuke that killing your friend is the only way to get the Mangekyou. This serves the dual purpose of having Itachi indirectly admit to the murder of Shisui (which paints him as a ruthless, power-hungry, murderous psychopath who will kill anyone to achieve his goals), and sets up a parallel between himself and Sasuke (who at this point in the story is finding himself drawn in by the power of the Curse Mark).
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With these points in mind, we get to the part of the story where Naruto and Sasuke do battle in the Final Valley for the first time. The whole time the reader is probably wondering if Sasuke and Itachi are truly cut from the same cloth and if Sasuke is capable of committing to Itachi's methods of gaining more power, which adds to the emotional weight of the fight itself. Then expectations are subverted when we see Sasuke in a position to kill a helpless Naruto at the end of the battle but spares him, which shows the reader that Sasuke is infact different than Itachi by having him avoid using Itachi's methods to gain power.