The author could try to justify it by some kind of mind - control or dark magic. Maybe an enemy has taken over Naruto's mind and forces him to do it without his full awareness.
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.
In fanfiction, Naruto might show his love for his mother through his memories. He could constantly recall the little time he had with her. For example, he might remember any advice or lullabies she gave him. These memories could fuel his actions, like when he faces difficulties, he might think about how his mother would want him to be strong and persevere.
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.
One possible reason could be a misunderstanding. Maybe some false information was spread about his mother, making Naruto believe she had abandoned him. Another reason could be related to the secrecy around his mother. If he grew up feeling alone and not knowing about her, he might develop resentment.
No. It is completely unacceptable. Incestuous relationships are not tolerated in society, and this goes against the basic moral principles that the Naruto series also upholds in its normal storylines. It is a very abnormal and inappropriate concept.
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.
In some fanfictions, Naruto might be made to hate his mother because of a misinterpreted prophecy or a false belief that she abandoned him. Maybe the author wants to create a more complex and angsty story line for Naruto.