It could also be due to a rift in their relationship over Boruto's ninja path. If Boruto decides to take a very different and unorthodox approach to being a ninja that Naruto doesn't approve of, such as using forbidden jutsu without proper understanding, Naruto might disown him in the fanfiction.
It could be due to a perceived betrayal. Suppose Naruto discovers some dark secrets within his family that he can't overlook. In his 'godlike' state, he might be more morally rigid and thus decides to disown them. For instance, if he finds out that his family was involved in some unethical ninja practices in the past, he would feel compelled to distance himself.
It could also be due to some external influence. Perhaps an enemy used some kind of mind - control or manipulation on Kushina. They made her think that Naruto was a threat, so she decided to disown him. In the Naruto universe, there are many powerful villains who could potentially do this kind of thing.
The disowning could also have an impact on the other characters. For instance, Hinata might be caught in the middle, trying to reconcile Naruto and Boruto. The other villagers might have different opinions about the situation. Some might side with Naruto, believing he had a good reason, while others might think Naruto is being too harsh on Boruto. This can add layers of complexity to the fanfiction's plot.
Perhaps Boruto disrespects the ninja way that Naruto holds so dear. In fanfiction, Boruto could be shown as being too rebellious, and Naruto, as a firm believer in the ninja values, would snap at him to correct his behavior. For example, if Boruto were to shirk his training or mock the importance of teamwork, which are fundamental to Naruto's view of being a ninja.
Maybe it's the high expectations that Naruto has for Boruto. Naruto is a hero, and he might unknowingly put a lot of pressure on Boruto to follow in his footsteps. Boruto could be scared of not being able to meet those expectations.
He could be curious about how other fans perceive the events in Naruto Shippuden. Fanfictions can offer unique interpretations and perspectives that might give him new insights.
Perhaps in fanfiction, Boruto is associated with a group or ideology that Naruto opposes. This could cause Naruto to view Boruto unfavorably. Also, if there are false rumors or misinformation spread in the fanfiction world about Boruto, Naruto might believe them and start to hate Boruto until the truth is revealed. And sometimes, it's just a way for the writer to make an interesting and unexpected plot twist.
One possible reason could be a lack of inspiration. Creating fanfic requires continuous creativity, and if Boruto has run out of new ideas, he might quit. Another reason could be time constraints. Maybe he has other important commitments in his life, like school or work, that leave him with no time for fanfic writing. Also, it could be due to negative feedback. If he received a lot of criticism on his fanfics, it might have discouraged him from continuing.
In the 'boruto kills naruto fanfiction', a motive could be a power struggle. Boruto might believe that Naruto's power is holding back the village's growth or his own potential. He could think that by eliminating Naruto, he can bring about a new era of power in the Hidden Leaf. Also, it could be out of desperation. If Boruto is in a situation where he believes that killing Naruto is the only way to save someone he loves, like his mother or a friend, he might take such a drastic step.
One common reason is envy. Naruto is so highly respected and has achieved so much. Boruto, in fanfic, might envy that and turn it into hatred. Another is the generational gap. Their values and ways of doing things could be different, leading to conflict and hatred in fan - made stories.