It could also be related to a prophecy or a pre - determined fate in the fanfic's plot. There might be some kind of dark prophecy that says Kawaki has to kill Naruto to prevent a greater evil. Or perhaps Kawaki is being controlled by an external force that has a vendetta against Naruto and uses Kawaki as a tool to carry out the murder.
In such a fanfiction, perhaps Kawaki is driven by some extreme ideology. Maybe he believes Naruto's way of peace is a weakness and decides to take him out to change the ninja world in a more radical way.
Many of Naruto's friends and allies would be filled with rage. Characters like Sasuke would probably seek immediate revenge on Kawaki. The other villages might also see this as a sign of instability in the Hidden Leaf and could either try to take advantage or offer help depending on their relationships with the Leaf. And the younger generation, like Boruto, would be deeply affected. He would have to deal with the loss of his father and might struggle with his own identity and purpose in a world without Naruto.
In a 'kawaki kills naruto' fanfic, the story would likely take a very dark turn. The characters would be forced to reevaluate their relationships and loyalties. Sasuke, for example, would be deeply affected as Naruto was his closest friend. He might be torn between seeking revenge and trying to understand what drove Kawaki to such an act. The new generation of ninja, like Sarada and Mitsuki, would also be involved in the aftermath. They might be caught in the middle of the conflict between Boruto and Kawaki, trying to find a way to restore peace while dealing with their own feelings of loss and confusion.
One common theme is friendship. Writers often explore how Himawari and Kawaki, who come from different backgrounds, form a bond of friendship. Another theme is growth. They both grow as characters, learning from each other.
Kawaki makes his appearance in Boruto manga at a specific point, but I can't recall exactly. You might want to check the official manga sources for the precise chapter.
One common theme is growth. Both Himawari and Kawaki are often depicted as growing both as individuals and in their relationship. Another theme is acceptance. Since they come from different backgrounds, fanfictions usually show how they accept each other's differences.
One great 'kawaki naruto fanfic' could be one where Kawaki has to learn a new jutsu from Naruto. In many fanfics, Kawaki is often portrayed as a complex character trying to fit into the Hidden Leaf Village. He might struggle with his past and Naruto could be the one to guide him through it.