Well, some writers might be interested in exploring alternate timelines or universes. In a different reality, they might think that Harry could be influenced by dark forces in such a way that he would do something as horrible as killing his parents. It could also be a form of creative rebellion against the established story. They want to break the rules and see where it takes them. And maybe, for some, it's a way to deal with complex psychological concepts, like how a person can lose their moral compass completely.
The existence of 'harry kills his parents fanfic' might be due to a desire for extreme creativity. Writers may want to see how far they can deviate from the original story while still using the same characters. It could also be a reaction to the over - saturation of the 'good Harry' stories. By creating a really dark and unexpected scenario, they are trying to offer something new. Additionally, it might be a way to explore the idea of fate and free will. If Harry is fated to be a hero, what would it take to break that fate and make him do something so atrocious?
Creative exploration. Some writers might want to see how the story would change if Harry had a darker side.
Some writers might want to explore the darker side of the character. They could be curious about how Harry would be if he was pushed to his limits and had to kill in self - defense or to protect others in a very extreme situation.
One reason could be to explore the dark side of human nature. By having Harry kill Ron, the writer might be showing how a good character can be corrupted. Another reason could be to create a shock factor. It's a very unexpected and extreme scenario that can draw readers in.
In such a fanfic, one possible reason could be extreme depression. Maybe he faced overwhelming losses like the death of his closest friends and felt he couldn't go on. Another reason might be that he was under some kind of dark magic influence that made him lose his will to live.
One possible reason could be that in the fanfic, the author might twist the story to make it seem like his parents' actions had unforeseen consequences that affected Harry negatively. For example, their fight against Voldemort led to Harry being an orphan and having to bear a great burden from a young age.
One reason could be exploring dark themes in the Harry Potter universe in a more extreme way. Some fans might be interested in delving into the psychological struggles that could potentially exist in that world, even if they are very dark. Another reason could be a form of catharsis for the authors, who may be dealing with their own internal issues and using the fanfic as a way to express their feelings in a fictional context.
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.
He could hate them because he felt they were absent during his difficult childhood. Naruto faced a lot of loneliness and bullying, and if he thought his parents could have prevented it but didn't, that would breed hatred.
One possible reason could be some past misdeeds by the parents. Maybe they were neglectful or made decisions that seriously affected Ciel's life in a negative way in the context of the fanfic.
One possible reason could be a misunderstanding. Maybe he discovers some secrets about his parents that he misinterprets. For example, if he finds out they were involved in some dangerous or morally ambiguous activities in their youth that he doesn't fully understand at first.