One possible reason could be a misinterpretation of his actions. Maybe he made a decision that others in the fanfiction misconstrued as betrayal, like his attempts to reach out to the enemy in an effort to bring peace but it was seen as siding with them.
In some fanfiction, new storylines might be introduced where Luke discovers some dark secrets about the Jedi Order he was a part of. His reaction to these secrets, such as trying to change or expose the Order, could be misread as traitorous behavior by those who are blindly loyal to the old ways of the Jedi. For example, if he found out that the Jedi had been involved in unethical practices in the past and he tried to make it public, some in the story might view him as a traitor.
In certain fanfiction, Luke's actions might be misinterpreted, leading to the idea of him being a traitor. For example, if he discovers a long - hidden truth about the Jedi and the Sith that challenges the established order. His attempts to expose this truth could be seen as traitorous by those who want to maintain the status quo. He might also be forced into situations where he has to work with the 'enemy' to save his friends or the galaxy, and this cooperation is wrongly construed as betrayal.
One reason could be that he's in deep meditation as a Jedi. Jedi often fast during their meditative periods to enhance their connection to the Force. So, in the fanfiction, this could be why he's not eating.
Luke might turn to the Dark Side if he experiences a great personal loss. Suppose his friends or family are killed and he blames the Jedi way for not being able to protect them. In his grief and anger, he could be lured by the power that the Dark Side offers. It gives him a false sense of control and the ability to seek revenge. Also, if he were to be exposed to powerful and alluring dark side artifacts or knowledge, it could corrupt his mind and make him question the teachings of the Jedi.
One common plotline could be that Luke is lured to the dark side by promises of power. The dark side characters might convince him that he can better protect the galaxy with their power. Another plotline is that he is wrongly accused of a crime against the Jedi Order or the Republic, which makes everyone think he's a traitor.
One possible motive could be extreme guilt. In the Star Wars universe, Luke has faced many challenges and made some tough decisions. If he were to make a huge mistake, like accidentally leading the dark side to a major victory, the guilt might be overwhelming. He's a character with a strong sense of justice and responsibility, and such a blunder could make him feel unworthy of living.
Another plot could involve a powerful Sith Lord who manipulates Luke. The Sith Lord could use Luke's insecurities, like his fear of not being strong enough to protect his friends. He could be tricked into believing that the Dark Side would give him the power he needs, and slowly turn to the dark side as a result.
One plotline could be that Luke discovers an ancient dance form on a remote planet that has some sort of connection to the Force. He then spends time learning and perfecting this dance, which in turn enhances his understanding of the Force in a new way.
Well, it could also lead to a secret meeting between them. Luke's call might prompt Vader to arrange a discreet encounter where they can talk more freely. During this meeting, Luke could try to persuade Vader to turn back to the light side, and Vader might be more receptive than expected because of that initial call. He might share some of his inner turmoil and regrets with Luke.
The plot could center around Luke's initial reasons for becoming a thief. It might be that he has lost his connection to the Force or has been exiled from the Jedi order. He then turns to thievery to make ends meet. As he steals, he could cross paths with old enemies or new characters who either try to stop him or recruit him for their own purposes.
It can create a lot of conflict. For example, it can split the loyalties of other characters. Some might still believe in Luke and try to prove his innocence, while others might turn against him, leading to internal battles within the group.