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.
It could be an unknown brother that was kept secret all this time. Maybe he was hidden away by some enemy of the village and trained to be a powerful adversary for Naruto.
Perhaps Naruto's brother could be a long - lost relative from a far - flung part of the ninja world. He could have been separated from Naruto when they were young due to a war or some political intrigue in the ninja clans.
One possible reason could be jealousy. Maybe his brother has some special abilities or gets more attention that Naruto feels he should have. For example, if his brother is really good at a certain jutsu and is always praised for it while Naruto struggles, that could lead to resentment.
Perhaps in the fanfiction, Naruto is tricked into believing his clan is a threat to the village. So, in an attempt to protect Konoha, he takes drastic measures and kills them. It could also be that a powerful and evil entity takes over Naruto's body, and using his strength, murders the clan while Naruto is powerless to stop it from within.
No, it doesn't seem to be a common concept. Usually, Naruto fanfiction focuses on various aspects like character development, new relationships, or different story arcs that are more in line with the general themes of the original series, which is not mainly about 'kills'.