Sasuke would likely be enraged. He and Naruto had a complex relationship, and Naruto was like a brother to him. He would probably seek revenge on Sakura immediately. Kakashi, as their teacher, would be deeply shocked and disappointed. He might struggle to understand how Sakura could do such a thing. The villagers would also be in turmoil. Naruto was their hero, and his death at Sakura's hands would cause a great deal of confusion and anger among them.
Hinata would be heartbroken. She loves Naruto deeply. Her world would shatter. She might try to find out the truth behind his death, not believing that his friends could do such a thing without a good reason. And she would probably hold a grudge against those involved.
Some characters might be shocked. For example, Sasuke could be surprised as he has his own complex relationship with both Sakura and Naruto. Kakashi, being their sensei, might be disappointed but also try to be understanding and help them through the situation.
Hermione would probably be devastated. She was Harry's closest friend and ally. She might be filled with rage towards Vernon and vow to seek justice. Ron would also be heartbroken and might want to take matters into his own hands against Vernon.
Well, if Naruto and Sakura kiss in fanfiction, the reactions would be diverse. Fans who have been rooting for them, known as shippers, would likely be overjoyed. They would see it as a validation of their long - hoped - for relationship. However, fans of other pairings, like Naruto and Hinata or Sakura and Sasuke, might not take it so well. They might feel that it goes against the 'canon' in a way. Also, some more purist fans might think that it's too out of character for the original Naruto and Sakura. There are so many different perspectives within the Naruto fan community.
Sakura might be worried about Naruto. She could try to comfort him and encourage him not to give up. She might also be angry at Jiraiya for abandoning Naruto, seeing how much Naruto has been through already. Kakashi, on the other hand, would probably step in more as a mentor. He has his own way of teaching, and he'd want to make sure Naruto doesn't go astray.
Sakura would probably be shocked. She knew the Akatsuki was powerful, and Naruto killing them all might make her see Naruto in a whole new light, perhaps as a more dangerous or powerful figure than she initially thought. Kakashi, on the other hand, might be worried about the implications. He would be proud of Naruto's strength but also concerned about the power vacuum and how it could affect the balance of the ninja world.
One possible reason could be a misunderstanding. Maybe Sakura is under the influence of some dark magic or mind - control in the fanfiction, which makes her lose her rational judgment and attack Naruto. Another reason might be a strong emotional outburst. For example, if she discovers something she believes Naruto has done wrong, but in fact it's a misinformation, and her anger gets the better of her, leading to her almost killing Naruto.
Sakura would probably be very surprised and might feel a bit of jealousy if she has some lingering feelings for Naruto. She could act cold towards Ino or Naruto for a while. Shikamaru, being Ino's friend, might tease her about it a lot, but also be a bit protective. Choji would likely be more concerned about how Ino feels and might offer her some snacks to cheer her up if she's flustered.
Some characters might be really surprised. Sakura, for instance, might be jealous as she has feelings for Sasuke and now Sasuke might show more interest in Naruto as a girl. Kakashi might be more protective of Naruto, knowing that this new revelation could bring some challenges. And the other ninja in the village could start to treat Naruto differently, either with more respect or in some cases, with unwanted attention.
Sakura might be a bit annoyed at first. She's used to Naruto being independent and always charging forward, and Gaara's overprotectiveness could seem like it's getting in the way. Kakashi, on the other hand, would probably be more understanding. He'd see the value in Naruto having someone looking out for him, but he might also try to encourage Gaara to let Naruto grow on his own a bit. Sasuke could have a very negative reaction. He might see it as Gaara trying to steal Naruto away from him in a sense, especially if he has his own unresolved feelings towards Naruto.