One possible reason could be extreme anger. If something really bad happened to his loved ones, like if Voldemort came back in a different way and killed all his friends and family this time, Harry might be driven to the edge and decide to destroy the wizarding world that couldn't protect them.
Another reason might be a false sense of justice. He could be misled by some false information or an evil character. For example, if a character convinces him that the only way to end all the discrimination and inequality in the wizarding world is to destroy it and start anew, he might believe it and act accordingly. He might think that by destroying the current power - hungry and corrupt system, a better wizarding world can be built from the ashes.
Maybe in the fanfic, Harry is under the influence of a powerful dark object. This object could be warping his mind and making him see the wizarding world as something that needs to be destroyed. It could be whispering to him all the wrong ideas, making him believe that the world is beyond saving through normal means, so destruction is the only option.
One possible reason could be the trauma he has faced. Harry has endured a great deal of pain, from the loss of his parents to the constant battles and threats in the wizarding world. He might want to escape all that and find peace away from it.
It could be that in the fanfic, Harry Potter discovers some deep - seated secrets about the wizarding world's power structure. Feeling betrayed, he decides to take extreme actions to expose and destroy it. For example, if he finds out that the Ministry of Magic has been using unethical means to control the wizarding population, he might use his influence and magic to bring it all crashing down. He could start by revealing the truth to the public, which could lead to chaos and the destruction of the established order in the wizarding world.
Well, one possible reason could be their ignorance of the magical world. Muggles, not understanding magic, might accidentally trigger events that lead to destruction. For example, they could unknowingly interfere with magical spells or artifacts that are crucial for maintaining the balance of the world.
Perhaps he feels disillusioned with the wizarding society. There could be issues like corruption or inequality that he can't bear anymore, so he decides to leave. For example, if the Ministry of Magic starts to misuse its power, Harry might not want to be part of such a world.
One possible reason could be that he's tired of the constant danger and politics in the wizarding world. After all he's been through so much. He might just want a normal, quiet life away from magic.
One reason could be his over - use of powerful magic spells. If he uses spells like the 'Geminio' spell (used to duplicate objects) on valuable items without any control, it could flood the market and make the items worthless, thus ruining the economy. For example, if he duplicates rare magical potions in large quantities, the potion - making industry would collapse as there would be no need to buy them at regular prices.
One reason could be that he values the authenticity of the wizarding world he lived in, and fanfiction often strays from that.
One possible reason could be that in some fanfics, the author wants to explore a different side of Harry. Maybe he's tired of the constant chaos and danger in the wizarding world. Another reason might be to show his independence from the typical wizarding narrative. He could be seeking a more normal life away from all the magic and politics.
One reason could be disillusionment. Maybe he discovers that the wizarding world is not as noble as he once thought, with a lot of hidden political intrigue and corruption. For example, the Ministry of Magic might be misusing its power, and Harry, being a symbol of justice, can't stand it anymore.
In some cases, he could be leaving in search of a greater purpose. He might feel that there are things he can learn or achieve outside the wizarding world that will ultimately help the wizarding community more. He may also leave because of some prophetic vision or a feeling that he doesn't belong there anymore, that his path lies elsewhere, away from the familiar magical realm he grew up in.