One motive could be that Itachi was trying to protect a greater secret or a more important person. He might have calculated that sacrificing Sakura's safety in that moment was worth it to keep something else safe. Or perhaps he had some sort of vendetta against Sakura or those associated with her. He could be using this as a way to get back at them. However, this is a very dark and immoral motive, but in the context of a fanfiction where such a scenario is presented, it's a possible explanation.
In this scenario, Sakura would be horrified. Her view of Itachi would turn from perhaps a neutral or even positive view (if she had any such view in the fanfiction world) to one of pure hatred. Their relationship would become one of enmity. Itachi using her as a shield would be a huge violation of trust and basic human decency, so she might even try to expose his actions to the other characters, causing a rift in the overall story's character dynamics.
Itachi might be doing this as a means of restraint to prevent Sakura from interfering in a dangerous situation. He may know that she has a tendency to rush into things without thinking, and this is his way of ensuring she stays put. Another idea is that it could be related to a cultural or ritualistic practice within the context of the fanfiction's world.
One possible plot could be an alternate universe where Itachi doesn't have his tragic backstory and he meets Sakura in a more normal setting, leading to a romantic encounter. Another could be a time - travel plot where Sakura goes back in time and somehow gets involved with Itachi.
Perhaps in this fanfiction, Itachi has a certain 'weight' of expectations or a mission that somehow involves Sakura. She might be struggling to deal with his decisions or the way he's affecting the situation around her. For example, if Itachi is on a path that goes against Sakura's beliefs, she could be fighting internally on how to react or whether to try and change his course. Another possibility is that Itachi has some form of power or ability that is creating a difficult environment for Sakura to operate in, and she is struggling to adapt or overcome it.
One possible plotline could be that Itachi and Shisui, being the skilled Uchiha they are, take Sakura under their wing to train her in some unique jutsu. Maybe they see her potential and want to help her reach a new level of power. For example, Shisui could teach her his body - flicker technique while Itachi imparts some of his knowledge about genjutsu. This would create an interesting dynamic as Sakura tries to adapt to their different teaching styles.
One possible plotline could be that Sakura discovers a new type of lemonade that has some sort of magical property. Itachi, being a powerful and mysterious figure, is interested in this lemonade for some reason related to his own goals or the well - being of his clan. Sakura, initially hesitant, decides to team up with Itachi to explore the secrets of the lemonade further. Maybe they find out that the lemonade can heal certain dark chakra - related ailments that are plaguing Itachi's clan.
One reason could be that he sees something special in Sakura. Maybe he admires her strength or her kind heart, and he doesn't want to lose her to someone else who might not appreciate her as much as he does.
Maybe Itachi, despite his stoic nature, starts to have an unexpected soft spot for Sakura. He could be secretly protecting her during her missions, which might lead to some tense and mysterious moments as Sakura tries to figure out who her mysterious protector is.
Perhaps in the fanfiction, Sakura and Itachi are in a police force that has to deal with international ninja disputes. Sakura, with her intelligence and strong will, and Itachi, with his strategic mind, work together to prevent a full - scale war between two large ninja clans. They face many challenges along the way, including distrust from their own comrades who are skeptical of their unorthodox methods.
The plot could involve Itachi noticing Sakura's isolation within her own team at some point. He decides to adopt her in a sense, like becoming a mentor to her. Itachi could share his insights on how to deal with powerful enemies and the sacrifices one has to make for the greater good. Sakura would then grow not only in her combat skills but also in her understanding of the complex world of ninjas. She might start to see things from a different perspective, similar to Itachi's, which could cause some friction and also growth within her relationships with Naruto and Sasuke.