Well, in some extreme fanfictions, it might be due to some kind of mind - control or dark magic that forces Naruto to do such a heinous act. But this goes completely against his character in the original series.
The original Naruto is about growth, friendship, and heroism. Naruto is depicted as a heroic figure who would never harm his family. In this fanfiction, the concept of Naruto as a 'god' and his act of killing his family introduce a very dark and uncharacteristic element. It changes the entire tone of the Naruto world from one of hope and perseverance to a much darker, more tragic one.
Well, it could be a dark corruption of power. Maybe in this fanfic, the 'god' power Naruto obtains warps his mind. He might see his family as a threat to his new - found godly status, fearing they'd try to stop him from using his power in certain ways.
This is a really dark concept. In fanfiction, such a scenario might be written to explore extreme emotional distress. Maybe the author wants to show how much pressure Naruto has endured secretly, and on his birthday, which is supposed to be a happy day, it all becomes too much to bear. However, in the original Naruto story, Naruto is a character full of hope and perseverance, so this is a very different take.
One possible plotline could be that Naruto is under some sort of dark magic or mind control. Maybe an enemy has found a way to manipulate his actions and make him turn on his friends. Another could be a situation where Naruto is tricked into believing his friends are actually enemies in disguise. For example, if there was a powerful genjutsu that made him see his friends as threats to the village.
If such a fanfiction exists, one possible reason could be a mind - control situation. Maybe an enemy uses some powerful genjutsu to make Naruto believe his sister is a threat, causing him to kill her accidentally. Another reason could be a misinformation plot where Naruto is tricked into thinking his sister has committed some unforgivable crime against the village.
I'm not aware of any official 'Naruto kills his sister' fanfiction. This goes against the general nature of Naruto's character who values family and friends. Fanfiction that involves such extreme and out - of - character scenarios might be very rare or non - existent in the mainstream Naruto fanfic community.
It would change it drastically. Naruto is known for his loyalty and friendship. If he were to kill his friends, it would shatter the image of the hero. His friends like Sasuke, Sakura, and Shikamaru are important to the overall story and relationships in the Naruto universe. Their deaths at Naruto's hands would create a huge rift among the other characters.
One possible reason could be that his newfound 'godlike' powers make his family feel estranged. They might not understand his sudden transformation and the magnitude of his power, which creates a sense of fear or discomfort, leading them to ignore him.