There could be. Fanfiction allows for a wide range of interpretations and alternate storylines. But it's a really extreme and disturbing concept considering the bonds that Naruto, Sakura, and Sasuke share in the official story. It would be a very different take on the characters, and most fans who love the original story would probably find it hard to accept.
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.
Sasuke would likely be a bit stunned at first. He's a complex character. But then he would become very protective. He has lost so much in his life, and he wouldn't want to lose Sakura or the baby. He might become more vigilant and even cut back on some of his more dangerous missions.
One common theme could be the exploration of trust issues. If Sasuke cheats on Sakura in fanfiction, it might show how Sakura deals with the broken trust. Another theme could be the impact on their relationship and how they might try to repair it or go their separate ways.
Sasuke might initially be shocked. He has always seen Sakura as someone connected to Konoha, so her leaving would be unexpected. He could be concerned about her safety, especially if she leaves without proper preparation or protection.
Their characters are also developed through their interactions with each other. For example, when they face a difficult situation together, like an attack on the village. Sasuke's combat skills combined with Sakura's medical knowledge create a strong partnership. This not only shows their growth as individuals but also how well they complement each other in the relationship.
One possible reason could be a sudden and extreme corruption of Naruto's character. Maybe he is influenced by some dark and powerful force that makes him turn against his friends. Another reason might be a misinterpretation of a prophecy or some kind of mind - control situation.
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.
It could be because of a dark power corrupting Sasuke. Maybe an ancient evil force takes over him and makes him see Sakura as an enemy. In some fanfics, this kind of external influence can completely change a character's actions. Also, it might be related to Sasuke's pursuit of power. If he believes Sakura is standing in the way of his ultimate power - seeking goal, he could be driven to kill her.
In general, a teacher - student relationship in fanfiction can be a complex and often controversial topic. For Sasuke and Sakura, it goes against their established dynamic in the original series. Their relationship in the Naruto series is more of comrades and later romantic partners on an equal footing. So, from a purist perspective of their characters, such a 'teacher - student' setup in lemon fanfiction might not be appropriate as it distorts their true relationship.