Another reason could be power. Harry might have been lured by the idea of greater power on the dark side. He had faced so many challenges and losses on the 'good' side, and the dark side could have promised him the ability to change things in a more decisive way, like being able to prevent the deaths of his loved ones.
Perhaps he was deceived. There might be a powerful dark wizard who managed to convince Harry that the dark side had a more 'just' cause. This wizard could have used false information about the history of the wizarding world and how the dark side was initially misjudged.
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.
Well, one possibility could be that Harry is drawn to the allure of the dark side because of a love interest there. Maybe a mysterious dark witch or wizard catches his eye and through that relationship, he starts to see the dark side in a different light. For example, they might introduce him to a more 'liberating' way of using magic according to the dark side's view, which slowly makes him question his previous beliefs.
Well, one possible plot development could be that Harry uses the cane as a symbol of his new dark power. Maybe he starts to use dark magic in ways that are unexpected, like using the cane to channel dark energy for attacks. He could also start to turn against his old friends, causing a rift in the wizarding world.
The Ministry of Magic could be the ones to betray Harry. They might try to use Harry as a pawn in their political games, and when Harry realizes this, he loses faith in the so - called good side. Voldemort then takes advantage of Harry's disillusionment and convinces him to join.
Perhaps Harry discovers some dark secrets about the wizarding world that make him lose faith in the good side. For example, he might find out that some of the so - called good wizards have been involved in immoral acts. This disillusionment leads him to think that the dark side might be a better option. As he joins them, he starts to abuse his power, using dark magic to get revenge on those who he believes have wronged him.
In 'Harry Joins the Dark Fan Fiction', perhaps Harry is deceived by a powerful dark wizard who convinces him that the dark path is the only way to truly protect the wizarding world. This could lead to him abandoning his previous values. He could be trained in dark arts, and as a result, he might become an outcast among those he once called friends. However, there could also be a plot twist where he realizes his mistake and tries to redeem himself.
Well, one possibility is that Harry is tricked into joining the dark side. Maybe Voldemort uses some dark magic to cloud Harry's mind and make him believe that the dark ways are the right ones. He could start using dark spells and turn against his friends.
It could be that the Dark Lord offers Harry something irresistible. Like a way to bring back his parents. Harry, being desperate to have his family back, might be tempted to join the Dark Lord in the fanfiction. After all, the love for his parents was always a strong driving force in his character, and this could be a major turning point that leads him to side with the Dark side.
Dumbledore could be seen as the main antagonist from Harry's new perspective on the dark side. Dumbledore represents all that is good in the wizarding world, and if Harry has turned to the dark, Dumbledore would be the one trying to stop him, to bring him back to the light. So in this context, Dumbledore becomes the opposition that Harry has to face.
She could join the dark side if she was manipulated. There might be a powerful and cunning character who preys on her insecurities and convinces her that the dark side is the right path. For example, they could make her believe that the Leaf Village's values are false and that the dark side has a more 'true' way of doing things. Maybe she was also influenced by seeing the dark side's seeming freedom from the strict rules of the village.