Sakura would be devastated. She has deep feelings for both Naruto and Sasuke. She would probably be angry at Sasuke for killing Naruto and at the same time feel guilty for not being able to stop it. She might even try to take revenge on Sasuke.
In some cases, Sasuke could be angry. He might see Naruto's request as a sign of weakness or as Naruto giving up too easily. He could lash out at Naruto, telling him to fight instead of asking for death. However, he would still be deeply troubled by the request and would probably struggle with it for a long time, constantly thinking about whether or not he should consider it.
Perhaps Naruto realizes that forcing Sasuke to stay would only create more resentment. By letting him escape, he hopes that Sasuke will have time to think and eventually return willingly. In the fanfiction, it could also be that Naruto has some sort of premonition that Sasuke's escape is necessary for a greater good in the future, like saving the village or the ninja world in a different way than if he were held back.
One possible reason could be that Naruto feels he has become a threat to the village or his friends in some way. Maybe he has some sort of dark power within him that he can't control and he believes Sasuke is the only one strong enough to stop him. Another reason might be related to a prophecy or a greater threat that Naruto thinks his death can prevent, and he trusts Sasuke to carry out this difficult task.
One idea could be that Naruto and Sasuke are somehow transported to the world of Kill la Kill. They might get involved in the fight against the Life Fibers, with their ninja skills being a unique addition. Maybe Naruto's Rasengan could be used to disrupt the Life Fibers in a new way.
One possible motive could be Naruto's extreme sense of loyalty and friendship. He might believe that by sacrificing himself, he can save Sasuke from the darkness completely. He could think that his death would shock Sasuke into realizing the error of his ways and bring him back to the side of good.
One reason could be Sasuke's thirst for power. If he believes Naruto is the only obstacle in his path to ultimate power, he might be driven to kill him. In some fanfictions, Sasuke may be influenced by a dark entity that twists his mind into thinking Naruto is a threat that needs to be eliminated.
One reason could be for power. Combining their strengths through marriage could make them an even more formidable force. Maybe Sasuke saw it as a strategic move to protect their village or to face some greater threat that they are not aware of yet.
In this fanfiction, the event could lead to a more nuanced relationship. Naruto's act shows his trust in Sasuke, and Sasuke might start to trust Naruto in return, albeit in a very convoluted way. They might communicate in a more unspoken manner. Naruto's letting Sasuke escape could also lead to a period of separation where they both grow independently, and when they meet again, their relationship has evolved. It could be a more mature, less one - sided relationship where they both respect each other's choices more.
One possible reason could be that Sasuke might have gone too far in his pursuit of power even after the war. He could have been a threat to the newfound peace in the ninja world, and Naruto, as the protector of peace, might have felt he had no other choice.