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.
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.
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.
This fanfiction changes the power balance within Konoha. Sakura was growing to be a capable kunoichi. Her death would mean a loss of potential power and knowledge for Konoha. Also, it could lead to a rift between Sasuke and the village. Instead of being a potential ally or a wayward ninja, he becomes a murderer, and Konoha would have to deal with him differently.
One possible motive could be that Sasuke has been consumed by his thirst for power. In the Naruto universe, Sasuke has often been shown to be willing to sacrifice a lot for power. In this fanfiction set in the Wave arc, perhaps he sees Sakura as an obstacle to his pursuit of power, either because she tries to stop him from following a dark path or he believes she will hold him back in some way.
There could be. Fanfiction allows for a wide range of interpretations and alternate storylines. But it's a really extreme and disturbing concept considering the bonds that Naruto, Sakura, and Sasuke share in the official story. It would be a very different take on the characters, and most fans who love the original story would probably find it hard to accept.
Yes, there might be some. Some fanfics could have a plot where Sasuke is under a powerful genjutsu or dark influence that makes him turn on Sakura, which would be a unique plot twist.
I'm not sure if there is such a specific fanfic. The idea of Naruto killing Sasuke and Sakura goes against the general themes of friendship and redemption in the Naruto series. Most fanfics tend to explore positive relationships and character growth rather than such dark and out - of - character scenarios.