She could also be amused in some cases. If the fanfiction has some really out - there but harmless ideas about Naruto, like him having a strange hobby that doesn't really change his fundamental good nature, Kushina might find it funny and just shake her head at the creativity of the fanfiction writer.
She could be angry if the fanfiction distorts Naruto's character or the story in a negative way. For example, if Naruto is portrayed as a bad person in the fanfiction, Kushina, as his mother, would not like it at all. But if she finds some positive and creative fanfictions, she might be impressed by the fans' imagination.
Well, one scenario could be that Kushina finds a fanfiction where Naruto has turned evil due to some dark force. Another might be that she discovers a story where Naruto is in a relationship that is completely out of character for him. Maybe she stumbles upon a fanfiction where Naruto has some really strange and overpowered abilities that don't fit the original Naruto universe.
Naruto would probably be really confused at first. He's used to a loving mother figure, not a yandere one. He might try to talk to her and understand why she's acting so strangely.
Sasuke might be affected as Naruto's closest friend. With Kushina back, Naruto could be more at peace, which might change the dynamic of their relationship. Sasuke could see a different side of Naruto, one that is more complete with his mother's presence.
Some villagers might be in shock. Kushina was an important figure in the village's history, being Naruto's mother. There would be those who would question Naruto's decision and lose faith in him. They might start to fear for their own safety, thinking that if someone like Kushina could be banished, they could be next.
Minato might be intrigued. He would be interested in seeing how the fans expand on Naruto's journey. Kushina would be delighted, seeing her son as the center of so many different tales.
Some fans would be extremely shocked and angry. Kushina is a beloved character, and Naruto slapping her goes against the loving relationship they should have as mother and son. They might feel that it's a disrespect to the original story's characters and themes.
In this type of fanfiction, the story could also focus on how the village might react differently to Naruto. Without Kushina's presence, which could potentially have been a positive influence, Naruto might face more hostility. His struggle to fit in and gain acceptance could be more arduous, and this could be what drives him to the brink of insanity. Maybe he would develop a more jaded view of the world and the people in it.
Kushina would be extremely worried. She would rush to Naruto's side immediately and start using her medical knowledge or any available resources to help him get better.
At first, Naruto would be shocked and confused. His world view would be shattered. But then, he would draw on his inner strength. He has overcome so many hardships before. He might feel a deep sadness, but also a sense of resolve. He would still believe in the power of love and friendship, and use that to face Kushina's betrayal. He could also use this experience to grow even stronger and more determined to prove himself and his beliefs.