Well, he would probably become much more cold and detached. He might lose his drive to protect his friends and the village in the same passionate way. His actions would be more mechanical, lacking the warmth and empathy that used to be his trademarks.
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.
Perhaps in the fanfiction, Naruto accepts his dark emotions with the help of his friends. His friends, like Sasuke, Sakura, and Kakashi, might encourage him to face his inner demons. They tell him that it's okay to have dark emotions and that everyone has them. Naruto then starts to accept his own, knowing that he doesn't have to face them alone. His friends' support gives him the confidence to embrace his whole self, including the darker aspects of his emotions.
One consequence could be that Naruto becomes more empathetic. By accepting his dark emotions, he can better understand the pain and struggles of others who also have their own inner turmoil. For example, he might be able to relate more to villains he once fought against. Another consequence is that he gains more control over his power. His dark emotions were probably holding back some of his true potential, and once he accepts them, he can use that energy in a more focused and effective way in battles.
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 consequence is that he might become more powerful. With his dark emotions out of the way, he could focus better on his training and unlock new abilities. Another consequence could be that he becomes more isolated. Since his dark emotions were a part of what made him relatable to others, without them, he might seem distant.
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.
His relationships would be in turmoil. Naruto not being aware of his emotions is like sailing a ship without a compass. In his relationships, especially with those close to him, he'd be lost. He might do things that seem out of character because he's not in touch with his feelings. For instance, when it comes to his sensei, he may not be able to show the proper respect or gratitude because he doesn't understand the emotions behind those concepts. His friends would constantly be trying to figure out what's wrong with him, and he'd be equally confused, leading to a lot of strife in his relationships.