She might be shocked at first. Sakura is used to Naruto being more passive when it comes to her hitting him, so when he suddenly stops her, it could catch her off - guard.
On the other hand, Sakura could also start to reflect on her own behavior. Realizing that Naruto is no longer willing to tolerate being hit might make her think about why she used to do it in the first place. Maybe she'll start to change and become more respectful towards Naruto, leading to a positive evolution in their relationship.
Sakura might be initially confused. She could be thinking about what she did wrong to make Naruto stop talking to her. Since they have been friends for a long time, this sudden change would be very puzzling for her.
One possible reason could be that Naruto has grown tired of Sakura's violent tendencies towards him. He might have reached a point where he decides to stand up for himself and not tolerate being hit any longer.
He might be really hurt. Naruto has always been an outcast and being called a demon again could reopen old wounds. So he could just stand there in shock and disbelief, his eyes wide with pain.
At first, Naruto would likely be extremely shocked. He has always considered Sakura a friend and teammate, so this would be a huge blow. He might question his own judgment in trusting her.
Naruto might initially be in denial. He's so used to having Sakura around and believing that they have a future together. So when she leaves for Sasuke, he might think it's just a temporary thing or that she's been influenced by something or someone. But as time passes and he realizes it's real, he would be deeply hurt. He might also start to question his own self - worth, thinking that he wasn't good enough for Sakura. And he might even try to win her back at first, but then slowly come to accept the situation and focus on his other responsibilities like being a ninja and protecting the village.
He could also get angry. After all, being misjudged like that is unfair. His anger could lead him to either lash out verbally at Sakura or he could leave the village in a fit of rage, feeling that he is not accepted even by his friends. This could then lead to a series of events where Naruto has to find his own way and Sakura realizes her mistake later on.
In some cases, Naruto might try to reach out to the Sakura he knows deep down. He could try to remind her of their past together, their training, and their shared experiences. He believes that the real Sakura would never do this, so he tries to break through whatever is making her act this way.
She may feel angry at first. Sakura is a strong - willed character. She might think that Naruto has made this decision without considering her feelings. In the fanfic, she could go on a rant about how he should have talked to her before making such a big decision.
He might be initially confused. Sasuke is not used to Sakura ignoring him as she has always been somewhat persistent in her attention towards him. So, he may be at a loss as to why she is suddenly acting this way.
She might be initially shocked. Sakura has always seen Naruto as a friend, so learning of his romantic feelings could completely change her perspective of him. It could make her re - evaluate all their past interactions.