One of the key parts of Tsukuyomi when Itachi uses it is that he's making Sasuke relive the psychological experience that drives Sasuke's core motives. Itachi is making sure that time won't dull the in-moment reality of the horror of what he experience when Itachi murdered their clan. One of the biggest issues is that time dulls memory of tragedy – and Japan experienced a lot of that in how it grappled with the aftermath of the atomic bombings, and when and how people were left struggling with them while others were quick to move on and try to forget the horrors.
When it comes to Itachi's underlying motive of making sure that Sasuke will NEVER forget how absolutely fucking awful that is, him constantly putting Sasuke back into that experience is central to what he's attempting to accomplish. It's the way that Itachi's making sure that his brother hates him for doing it, and won't ever be able to just numb the pain of the experience. That's connected to the same part of the underlying concerns that
Shisui had about the misuse of his own Sharingan which he was going to attempt to use to stop his clan from executing a Coup, and how he trusted that task to Itachi in his absence, and the whole cycle of development that's built off of that.
Additionally, if you want a SHITTON of information on what's going on with Sasuke & the Uchiha clan, watch/read
Tekkonkinkreet. Black has
an alternate personality of a minotaur who embodies embracing the power of darkness, madness, revenge, vengeance, and violence over the possessive ownership of the part of the city that he protects – and the minotaur's name is Itachi. He starts to appear when they take his little brother White away from him, and in the scene where they are separated
Black is depicted standing in top of a telephone pole, with a crow circling around with him – both of which are critical pieces of Itachi's own iconography in
Naruto, with
the telephone pole being the thing Sasuke sees on his way home just at the start of the Uchiha massacre. It's following those exact same underlying motivations in attempting to create someone strong enough to do what's necessary to protect their village.
Also, FWIW
Remake intentionally introduced the pronoun differentiation in Sephiroth's speech patterns to help emphasize things about his psychological state, that's why
Remake's use of Sephiroth using "ore" as his pronoun for his pre-Nibelheim self and also the self who talks to Cloud about wanting to work together at the Edge of Creation is narratively worth thinking about in this sort of a context, because that's not the same as the "watashi" pronoun that he uses whenever he's being the insane object of hatred that Cloud is seeking vengeance against who annihilated all of Nibelheim. One of the more interesting underlying questions to look at in the game is how Sephiroth exists in a way that always serves as a motivator for Cloud to force him to be the things that Cloud never accomplished – which is being a SOLDIER first and foremost. In
Remake it's really important to look at all of the dialogue that Sephiroth has with Cloud in the little memory glitches. It all focuses on making Cloud relive all of his weaknesses when his mom was killed, and gaining the strength to stop being too weak to save the people that he cares about.
This is ultimately rooted in the core reason Cloud wanted to join SOLDIER, which was so that he would never be too weak to prevent Tifa from getting hurt again like she did out at the bridge on Mt. Nibel when they were kids after Tifa's mom died, and Cloud was blamed for her getting hurt and slipping into a coma for a week. Throughout the game, Sephiroth is constantly forcing Cloud into a situation where Cloud is tempted to hide who he really is just like he did back when he returned to Nibelheim, and then slowly HAS to come to terms with who he actually is and define is real self against the version of himself that Sephiroth is using to achieve his own ends. On top of that, Sephiroth is constantly putting Cloud in situations where the thing(s) that he cares most about in the entire world are the things that he is utterly powerless to protect. That's why after defeating Sephiroth at the very end of the game, Cloud leaps up to prevent Tifa from falling off of the cliff ledge. It's why he explicitly asks Cloud what he treasures most, so that he can take it away from him in
Advent Children because he's making Cloud regain that strength.
The point here is that from a narrative perspective –
Sephiroth and Itachi serve EXACTLY the same purpose to Cloud & Sasuke respectively. In Itachi's case, you just got to see more about the sociopolitical game that he's a part of attempting to overcome from his own point of view, whereas he have literally NEVER seen the narrative of events from Sephiroth's point of view – only from the narrative that he openly provides to the main character. The closest thing we have to that motive are the things that Kadaj talks about with Rufus in
ACC, where they are beings existing on a Planet that is constantly attempting to erase them, so they hate it and want to destroy it – which is just the exact same reason that the Planet wants to destroy them. It's the inherent tragedy of creating a character who's role is designed to be that of a monster.
Whether or not they are intentional or incidental motivators for the hero to overcome depends on the story, but that's why in the Japanese narrative the most important part of that is pushing to emphasize how the two opposed forces are close to identical to one another, which is why the motivations that they give work – they are a perfect dark reflection designed for the hero to recognize themselves in and to surpass in a way that the dark individual isn't able to. That's why Japanese stories usually provide all of the careful psychological motivation and allow them to exist as an ambiguous motivator, while the end is effectively achieved ONLY when they are exclusively seen as an absolute monster.
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