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.
The power balance would be severely disrupted. The Rebellion would lose one of its most important leaders, and the Empire or any other dark side - aligned forces would gain a significant advantage. Her knowledge of the Rebellion's strategies and bases could be used against them.
In fanfiction, Anakin joining the Dark Side often has complex motives. It could be due to his fear of losing Padmé. The Dark Side might seem to offer him a way to save her, so he succumbs to its power.
One possibility could be that Batman's sense of justice becomes warped. He might start using more extreme and unethical methods to 'clean up' Gotham. For example, he could be more brutal with criminals, foregoing his usual no - kill rule. His relationships with his allies would also be severely strained. Alfred would be deeply disappointed and the Bat - family might split apart as they try to oppose his new, darker path.
One possibility is that she becomes disillusioned with the Rebellion. Maybe she discovers some dark secrets within the Alliance that make her question their motives. For example, if she finds out that the Rebellion has been using unethical tactics in the fight against the Empire. This could push her towards the dark side as she feels betrayed by those she once trusted.
It could be a new, powerful dark side user created specifically for the fanfiction. This character might have a vendetta against the Skywalker family and sees turning Leia as a way to get revenge on Luke and Anakin. They could use Leia's insecurities and her desire to protect her loved ones against her.
One possible plotline could be that after facing great losses and betrayals, Leia starts to question the light side's values. Maybe the death of a close friend or the destruction of something she holds dear makes her vulnerable to the temptations of the dark side. She might then begin to explore the power that the dark side offers as a means to gain control and revenge.
Well, perhaps Harry gets tired of being constantly in danger and feeling unappreciated in the wizarding world. He might be lured by the Dark Side with promises of power and safety. Voldemort could offer him a place by his side, showing him a different view of magic that seems more appealing in a warped way.
In fanfiction scenarios where Naruto turns to the dark side, it could start with a personal tragedy. For example, if Hinata was killed by someone from the Leaf Village under false accusations. This could shatter Naruto's belief in the village and its values. He would then be vulnerable to the influence of the dark side. Once on the dark side, he could use his charisma to recruit others who have also felt wronged by the ninja world. He might form his own group, with different rules and goals from the traditional ninja villages. His journey to the dark side would not only change him but also send shockwaves through the entire ninja world, as he was once seen as a symbol of hope and a force for good.
In this fanfiction, it could be that Rey has some personal losses that make her turn. She might lose someone dear to her and in her grief and anger, she decides that the dark side can give her the strength to prevent such losses in the future. She then starts learning the ways of the dark side, which changes her relationships with her friends.