In this type of fanfiction, Fem Sasuke could be motivated by a sense of betrayal. Maybe she thought Naruto had promised her something or would support her in a certain way, but when he didn't, she felt so angry and hurt that she took his life. Or she could be under the influence of an evil organization that has convinced her that Naruto is the enemy and must be eliminated for the greater good.
Perhaps Fem Sasuke in this fanfiction has a different view of the world. She might believe that Naruto is a threat to her goals or to the future of the ninja world she envisions. It could also be that she has some deep - seated emotional pain that she blames Naruto for, and that leads her to kill him.
Jealousy could be a motive. If Fem Sasuke sees Naruto getting all the attention and praise while she feels overlooked, she might betray him out of spite.
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.
One possible motive could be power. In some fanfics, Sasuke might be driven by an insatiable thirst for power and see Naruto as an obstacle. Another could be revenge, perhaps believing Naruto has wronged him in some way in that alternate story. Or it could be a matter of being corrupted by dark forces, which make him turn against Naruto.
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.
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.
A possible motive could be a creative exploration of the 'what - if' scenarios. Maybe the writer wants to see how the story would unfold if Sasuke completely lost his way. Another motive could be to create a sense of high - stakes drama. Since Sakura is an important character, her death at Sasuke's hands would be a huge shock.
Perhaps in a fanfiction where the world has been completely turned upside down. If Sasuke had been tricked into becoming an unstoppable villain who was about to destroy the entire ninja world, and Sakura was somehow aiding him (maybe under a genjutsu), Naruto might feel he had no choice but to stop them by killing. Another reason could be a broken heart. If Naruto loved Sakura deeply but she constantly rejected him and chose Sasuke, and then Sasuke and Sakura betrayed him in some way, his anger and pain could lead to such a drastic act.
One common theme is friendship. Since Naruto and Sasuke's relationship is so central in the original series, in fem versions, their friendship or evolving bond is often explored. Another theme is rivalry, but with a different female - centered dynamic. For example, how they compete in a world that might have different expectations for female ninjas.
One possible reason could be jealousy. Maybe fem Sasuke is jealous of Naruto's natural charisma or his ability to make friends easily. Another reason might be related to their different goals. If fem Sasuke has a very different view of how to save or change the ninja world compared to Naruto, it could lead to hatred.
Also, it allows for a deeper exploration of gender roles in the Naruto universe. A fem Naruto might face discrimination or different opportunities compared to male Naruto. How Sasuke responds to this, whether he becomes more protective or if it makes him view her differently, is a fascinating aspect. It gives readers a chance to see the Naruto world through a different lens, one that challenges the traditional male - centered narrative.