Another reason could be a misunderstanding. There could have been a situation where Sasuke misinterpreted Sakura's actions as a threat to his own safety or his goals. For example, if Sakura tried to stop him from doing something he thought was crucial for his revenge, he might have overreacted and nearly killed her.
One possible reason could be that Sasuke was under some kind of mind - control or dark influence at that moment. Maybe he was influenced by an enemy's jutsu that made him lose his sense of self and turn on Sakura.
It might be related to Sasuke's inner turmoil. He was always conflicted between his revenge and his relationships. In this fanfic, perhaps he saw Sakura as an obstacle to his path of revenge, even if just for a brief moment. His intense focus on getting stronger and achieving his goal of killing Itachi could have made him lash out at her.
Perhaps Sasuke was testing Sakura's loyalty or strength. He may have thought that if she couldn't withstand his aggression, she wasn't worthy of being in his life. Also, his own inner demons, like the hatred he carried from his past, could have flared up and led him to almost kill her without fully realizing what he was doing.
It could be because of a dark power corrupting Sasuke. Maybe an ancient evil force takes over him and makes him see Sakura as an enemy. In some fanfics, this kind of external influence can completely change a character's actions. Also, it might be related to Sasuke's pursuit of power. If he believes Sakura is standing in the way of his ultimate power - seeking goal, he could be driven to kill her.
Sakura might be in shock. She probably couldn't believe that Sasuke, someone she cares about deeply, would try to kill her. So she could be frozen in place for a moment, unable to react immediately.
Sakura may initially be paralyzed by fear, but then her love for Sasuke and her own determination to survive could kick in. She might try to counter - attack in a non - lethal way, hoping to subdue Sasuke rather than harm him. She could also call out to their shared memories in an attempt to stop him from actually killing her.
One possible reason could be a sudden and extreme corruption of Naruto's character. Maybe he is influenced by some dark and powerful force that makes him turn against his friends. Another reason might be a misinterpretation of a prophecy or some kind of mind - control situation.
Maybe to explore different character arcs. The writer might want to see how Sakura would develop as a character if she strayed from her loyalty to Sasuke. It could also be a way to introduce new characters or relationships into the Naruto fanfic world.
One possible reason could be Sasuke's focus on his revenge. His thirst for vengeance against Itachi might make him blind to Sakura's feelings and neglect her as he is constantly preoccupied with getting stronger to achieve his goal of revenge.
One possible reason could be that Sakura has finally grown tired of Sasuke's aloof and cold behavior towards her. She might have realized that she has been constantly chasing after him while getting little in return, so she decides to ignore him as a form of self - preservation.
Maybe in a very dark and twisted fanfic, Naruto could be possessed by some evil force. This would override his normal sense of self and his love for his friends, causing him to harm them.
One possible reason could be that he overestimated his abilities during a battle in the Wave arc. Maybe he took on an enemy that was much stronger than he thought and got fatally injured. Another reason could be that he sacrificed himself to save Naruto or someone else. It's also possible that there was some sort of betrayal within the group they were with, and Sasuke was caught off - guard and killed.