One possible motive could be jealousy. Maybe she was jealous of Naruto's growing power and popularity within the village. Another reason might be related to some dark family secret or a curse that forces her to take his life against her will.
It's possible that she was in a state of mind -altering situation. For example, she could have been under the influence of some dark jutsu that made her see Naruto as an enemy. Maybe she was possessed by some evil spirit that wanted to destroy Naruto because he was seen as a key to the future peace of the ninja world. Also, it could be that she had some deep - seated hatred towards Naruto due to a misunderstanding in their past that was never resolved and it finally boiled over into this extreme act.
One possible motive could be if Sasuke goes too far in his pursuit of power and becomes a threat to the entire ninja world that even Naruto can't reason with. Maybe Sasuke starts to kill innocent people without hesitation, and Naruto, as the protector of the village and the one who believes in peace, sees no other option but to stop him by killing. Another motive could be some kind of dark magic or mind control that makes Sasuke an uncontrollable force of destruction, forcing Naruto to take drastic measures.
A possible motive could be that Naruto discovers some dark secret about Minato that makes him feel betrayed. For example, if he finds out that Minato was involved in some unethical experiments that led to the suffering of many people. Or perhaps Naruto is tricked by a powerful villain who manipulates his emotions and convinces him that Minato is the root of all his problems.
One possible motive could be mind control. Maybe an enemy manages to take over Kakashi's mind and forces him to harm Naruto. Another could be a misunderstanding. For example, if Naruto was thought to be possessed by a powerful and dangerous entity that Kakashi believed could only be stopped by killing Naruto.
One possible motive could be jealousy. Maybe the sister is jealous of Naruto's popularity, his strength, or the attention he gets from others in the village.
One possible motive could be a complete breakdown of the peace they had worked so hard to build. Maybe an outside force manipulates Gaara's mind, causing him to turn on Naruto. Another could be extreme jealousy over Naruto's popularity or power.
One possible motive could be a misunderstanding. Maybe Naruto was tricked into believing that Sasuke's sister was an immediate threat to the village. Another could be that she was somehow involved in a dark plot against Naruto's friends, and Naruto felt he had no other choice to protect them.
One possible motive could be a dark magic or mind - control situation. Maybe an enemy has the power to control Naruto's actions and forces him to kill Sasuke against his will. Another could be a miscommunication where Naruto believes Sasuke has done something unforgivable, like harming the village in a way that Naruto can't overlook.
It could be an original character created just for the fanfiction. Many fanfiction writers like to introduce new characters to add their own unique twists to the story.
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.
One possible motive could be that Sakura is brainwashed. Maybe an enemy uses some kind of mind - control jutsu on her and she becomes a puppet to kill Sasuke. Another could be extreme jealousy. If she felt that Sasuke was slipping away from her in some way, in a very dark and twisted version of the story, it could lead to her killing him.