Perhaps he becomes disillusioned with the Jedi Order. He might see their rules as too restrictive and the dark side as a way to break free and gain true freedom. He could start to question the Jedi's non - aggression policy and think that the dark side's more aggressive approach is better for dealing with the galaxy's threats.
One possible scenario could be that Luke is lured by the power promised by a powerful dark side figure. Maybe he's faced with a situation where he believes the dark side can save someone he loves, like if Leia was in mortal danger and the only way he thought he could save her was through dark side powers.
Maybe in the fanfiction, Luke witnesses something so devastating that he loses faith in the Jedi way. For example, if the Jedi Order fails to prevent a great catastrophe. This could push him towards the Dark Side as he seeks a more radical solution to bring order. He might start using aggressive and unethical means, like Vader did, believing it's for the greater good.
Well, in this type of fanfiction, perhaps Harry could be lured to the dark side by a new, extremely charismatic dark wizard or witch. This new character might offer Harry power, knowledge, and a sense of belonging that he feels he lacks in the wizarding world. They could manipulate Harry by appealing to his insecurities, such as his status as an orphan and his feelings of being different from other wizards. And then there's the idea that the dark side might seem like a more efficient way to achieve his goals. If Harry is frustrated with the slow progress of fighting Voldemort through traditional means, he could be tempted to use dark magic to end the threat once and for all.
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.
In some fanfictions, Luke might fall to the dark side after being betrayed by his closest friends. For example, if he discovers that they have been hiding important information from him that he thinks could have saved lives. This sense of betrayal could push him towards the dark side as he feels abandoned by the light. Also, a powerful dark side user could use mind control techniques on Luke, slowly turning him over time.
One possible scenario could be that Leia is manipulated by a powerful dark side user. Maybe through false promises of power or a way to protect her loved ones in a distorted way. Another could be that she becomes disillusioned with the light side due to some great loss or perceived betrayal within the light side forces.
Palpatine could still be the main antagonist even if Luke turns to the dark side. He might try to use Luke as a more powerful weapon against any remaining Jedi or the Rebel Alliance. Palpatine would see Luke's newfound dark side power as a means to further his own control over the galaxy.
In the fanfiction, Luke could be influenced by his family ties to the dark side. If he discovers some hidden connection to Vader or Palpatine that he didn't know before, it might make him question his place on the light side. He could think that joining the dark side is a way to understand his true heritage better and gain more power to protect those he loves in a different way than the Jedi would allow.
One possible trigger could be a sense of betrayal. If Ezra felt let down by his friends or mentors, it might push him towards the dark side. For example, if they didn't believe in him during a crucial mission or made decisions that he thought were wrong.
If Luke Skywalker turns to the Dark Side in fan fiction, one possibility is that he could become a powerful and feared Sith Lord. He might use his Jedi skills in a perverted way, like using the Force to dominate rather than protect. His relationship with his friends would be severely strained. Leia, for example, would be heartbroken and might try to bring him back, but it would be a very difficult task. Han Solo would also be in a tough spot, having to choose between his loyalty to Luke and his own sense of right and wrong.
One possible plot could be that Luke is lured by the power the dark side offers. Maybe he gets frustrated with the slow progress of the light side in bringing order to the galaxy. He might see the Sith as more efficient in getting things done. For example, if the Republic is still facing numerous threats and the Jedi are tied by their code not to take extreme measures, Luke could be tempted to cross over.