Maybe the writer wants to be different and create a shock factor. By making Harry a dark wizard, it goes against the common perception and grabs attention.
Maybe for the sake of novelty. People might be tired of the traditional good - vs - evil story in the Harry Potter world and want to explore a completely different take on the character. It could also be an attempt to show how power can corrupt, even someone like Harry who was initially on the side of good.
One great 'Dark Wizard Harry Potter' fanfic is 'The Dark Lord's Heir'. In this story, Harry discovers a hidden heritage that ties him to the dark side in unexpected ways. It has a really interesting plot that twists the original story's elements. Another one is 'Harry Potter and the Dark Arts Reborn' which delves deep into the dark magic that Harry starts to explore, creating a more morally ambiguous world compared to the original books.
One reason could be that Harry has had enough of the wizard world's troubles. He might want a normal life away from magic. After all he's been through, like fighting Voldemort and losing many friends, he may just be tired of the whole wizarding business.
He might hide it because he doesn't fully understand his own fascination. Harry has always been on the side of good, but perhaps there are elements of the dark side that seem alluring in a strange way. He could be afraid that exploring these feelings would lead him down a dangerous path, so he keeps it a secret.
One reason could be the strict control by the official copyright holders. They might be worried about fanfic diluting the official brand or misinterpreting the original story. Another factor could be some fans' purist view. They only want to stick to the original works by J.K. Rowling and consider fanfic as something not in line with the 'true' Harry Potter universe.
I think it's a bad take. In the official Harry Potter lore, Harry has always been on the side of light. He was the one who faced Voldemort, the ultimate dark wizard, multiple times and defeated him. To make Harry a dark wizard in a fanfic completely distorts the well - established character arcs and the overall positive message of the series.
One major theme is the allure of the dark side. It shows why some wizards might be attracted to dark magic and the lifestyle of a dark wizard. There's also the theme of corruption. Dark magic often corrupts those who use it, and these fanfics explore how that happens. Additionally, the conflict between good and evil is still present, but often with a more nuanced look at the dark wizard's perspective and what drives them to be on the 'dark' side.
It could also be that the fanfic might interfere with the way he wants his story to be remembered. His adventures at Hogwarts were real and important to him, and when fanfic changes the narrative, it's like his story is being disrespected. For example, if a fanfic portrays him as cowardly when he was actually brave, that would surely make him dislike it.
One possible reason could be a miscommunication. Maybe Hermione accidentally reveals a secret of Harry's that he didn't want known, and this causes him to feel betrayed and start to hate her. Another reason could be jealousy. If Hermione gets more attention or praise for her magical abilities than Harry in the fanfic, he might develop feelings of envy and hatred. Or perhaps there's a love triangle situation where Hermione is involved with someone Harry doesn't like, and that causes his hatred.
One possible reason could be extreme despair. Maybe in that fanfic's alternate universe, Harry has lost all his friends and loved ones in a very tragic way, like a second wizarding war that was far more brutal and unforgiving. He might see no way out of the emotional pain and thus choose to end his own life.