Well, it could be that he has lost all hope. In the fanfiction world, if the situation becomes so dire that he sees no way out for himself or the people he loves. For example, if Voldemort manages to gain complete control and Harry Potter thinks his sacrifice by taking his own life might be the ultimate solution to break Voldemort's power. He might also be suffering from post - traumatic stress after all the battles and losses, leading him to this extreme decision.
Some authors might create it as an exercise in 'what - if' scenarios. They want to break the established narrative and see how different characters would react if Harry was killed so early. Also, it could be a way to explore the Dursleys' characters more deeply, showing just how far their hatred for Harry could go.
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.
One possible reason could be a misdirected spell during a battle. In the heat of a fight, Harry's spell could go awry and accidentally kill Malfoy. Another reason might be if Malfoy was under the control of a dark wizard and was about to do something truly terrible, and Harry had to stop him at all costs, including taking his life.
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.
In some fanfictions, the power dynamics might be shifted. Maybe Lucius has become more powerful and is on the verge of destroying the wizarding world as Harry knows it. Harry, seeing no other way to stop him, decides to kill Lucius. It could also be a way for the writer to explore the darker side of Harry's character, showing what he could become when pushed to his limits and faced with an unrelenting enemy like Lucius.
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.
He could have been influenced by the Muggle world. Having grown up with Muggles, he might have seen the power of guns and thought it could be a more straightforward solution compared to complex magic spells. Also, he might have wanted to prove that Muggle technology could be just as effective in the wizarding world's battles.
Well, it could be a story where Harry is tricked into believing that Muggles are responsible for some great harm to his loved ones in the wizarding world. So, in his rage and misdirected sense of justice, he starts targeting Muggles. But this is a very un - Harry - like behavior as the real Harry is all about protecting the innocent, whether they are wizards or Muggles.
One reason could be that the official Harry Potter canon is so extensive and detailed that it overshadows fanfiction. The original story has such a rich world, characters, and plotlines that it becomes the main focus for many fans, leaving less room for fan - created stories.
One reason could be safety. Some fanfiction authors might think that Hogwarts is too dangerous for Harry, so they make him homeschooled to keep him out of harm's way.