If in a Sasuke fanfic, one possible reason could be unrequited love. If Sakura feels that Sasuke will never return her love and she has exhausted all hope, she might sink into despair. Another reason could be if she feels she has failed Sasuke in some way, like not being able to protect him or help him in his goals. Maybe she blames herself for his continued pain or suffering and sees no way out but to end her own life.
Well, in a Sasuke - centered fanfic, Sakura might kill herself if she is constantly rejected by Sasuke. Imagine she has been trying for years to get close to him, to be a part of his life, but he keeps pushing her away. This continuous rejection could break her spirit. Also, if there is some sort of misunderstanding. For example, she might think Sasuke blames her for something that she didn't do, and she can't bear the thought of him hating her, so she takes her own life. Additionally, if she is in a situation where she feels she has lost everything related to Sasuke, like his trust, his friendship, or any chance of a future with him, she might consider suicide as an escape.
One reason could be to create shock value. By having two main characters kill another important character, it grabs the reader's attention. Another could be to explore a very dark and alternate version of the Naruto universe. Maybe the writer wants to show how far characters could be pushed in extreme situations.
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.
Maybe to explore different character arcs. The writer might want to see how Sakura would develop as a character if she strayed from her loyalty to Sasuke. It could also be a way to introduce new characters or relationships into the Naruto fanfic world.
One possible reason could be Sasuke's focus on his revenge. His thirst for vengeance against Itachi might make him blind to Sakura's feelings and neglect her as he is constantly preoccupied with getting stronger to achieve his goal of revenge.
One possible reason could be that Sakura has finally grown tired of Sasuke's aloof and cold behavior towards her. She might have realized that she has been constantly chasing after him while getting little in return, so she decides to ignore him as a form of self - preservation.
Perhaps they leave because they feel restricted in the village. They want to explore the world and grow as ninjas outside the boundaries and rules of the village. They might also want to learn from different ninja cultures and techniques that are not available in their own village.
Sasuke might believe that he's a danger to Sakura. Given his associations with dangerous elements and his own dark powers, he may feel that getting close to Sakura would put her in harm's way. Therefore, he rejects her to protect her. This is a common theme in many fanfics where Sasuke is portrayed as having a strong sense of self - sacrifice when it comes to Sakura.
One reason could be jealousy. If in the fanfic there's another character who Sasuke is close to, Sakura might be rude out of envy. Another reason could be that she feels betrayed by Sasuke in some way, like if he made a decision without considering her feelings. Also, it could be part of her own character development, starting off rude to show her flaws and then changing later.
Maybe in the fanfic, Sasuke has some personality traits that become more difficult for Sakura to handle over time. For example, his brooding nature might start to feel like emotional unavailability rather than just his usual self. This could lead to her regret.
Perhaps in the fanfic, Sasuke was being pushed to his limits. He was so intent on protecting the village from a powerful threat that he didn't realize Sakura was in a dangerous position. His Sharingan might have malfunctioned in some way, causing him to misinterpret Sakura's movements as hostile. Or, there could be a situation where an enemy was using Sakura as a shield, and Sasuke's attempt to strike the enemy ended up harming Sakura instead, all in a split - second decision.