To make it believable, the author could explore a dark AU (alternate universe). Maybe in this AU, Draco has been pushed to the extreme by external forces. For example, if his family was in mortal danger and the only way to save them was to eliminate Harry. It would also require a significant change in Draco's character development, showing how he became so desperate.
For such a plot to be believable in a fanfic, the writer would need to do a lot of groundwork. They could start by changing the power dynamics in the wizarding world. If, for instance, Voldemort had won much earlier and Draco was under his complete control. This would put Draco in a position where he might be forced to kill Harry. Additionally, the author could explore Draco's psychological state, perhaps he was consumed by jealousy and fear, which could drive him to commit such an act.
Well, first, the fanfic could set up a complex backstory. Suppose there was a powerful dark magic influencing Draco, forcing him to act against his will. Also, the relationship between Harry and Draco could be completely different from the original. Maybe they had a huge misunderstanding that led to intense hatred. The author would need to rewrite the character dynamics carefully to make it seem possible for Draco to kill Harry.
Well, perhaps the dragon is terrorizing a village near Hogwarts. Harry, being the brave hero, decides to take it upon himself to stop the dragon. He uses his magic skills, like the Patronus charm to disorient the dragon, and then a powerful blasting spell to finish it off.
Maybe in that fanfic's plot, there was a misunderstanding that got out of hand. Suppose Harry was framed for something terrible, and Hermione, believing the false evidence, felt that she had to take drastic action to protect the wizarding world from what she thought was a great threat, so she kills him. But later she realizes her mistake.
Well, perhaps it could be a fanfic where they are on a secret mission together. After getting dirty during the mission, they find a single shower area in an old, abandoned building. They have to take turns, but there are some magical artifacts in the shower that start causing strange things to happen. Harry might be the first to notice something odd, Hermione would try to analyze it, and Draco would be reluctantly involved.
The plot could involve Harry and Draco being accidentally transported to Burdel during Redada. They have to find a way to get back to their own world while also dealing with the chaos in Burdel. For instance, they might encounter strange creatures or dark wizards that are taking advantage of the Redada situation.
One possible way in a fanfic could be that there was a misunderstanding. Hagrid might have been accidentally given a poisoned item by Harry that was meant for someone else. But still, it's a disturbing concept as Hagrid is a beloved character. It could also be that the fanfic was exploring a parallel universe where the characters' personalities and relationships are distorted, leading to this tragic event.
The author should also consider their personalities. Harry is brave and sometimes impulsive, while Hermione is intelligent and logical. Their relationship could grow as they learn from each other. For example, Harry could learn to think more before acting from Hermione, and Hermione could learn to be more spontaneous from Harry. Also, adding in some conflict related to their different approaches in life can make their relationship more real. Maybe they have an argument about how to handle a situation at Hogwarts, but then they make up and realize they care too much about each other not to be together.
It could be a story about a bet. Maybe Draco dares Harry to wear a dress, and in the process, they start to see each other in a new light and develop an unexpected friendship or more.
One possible reason could be for a power play. Draco, being from a Slytherin family, might want to show his dominance over Harry, who is seen as a hero in the wizarding world. By tying him up, he could be trying to intimidate Harry or gain some sort of advantage in their ongoing rivalry.
The plot could involve a time - travel element. Harry and Draco are sent back in time to the founding of Hogwarts. There, they have to work together to preserve the future of the wizarding world. As they interact with the founders, they learn from each other. Draco's knowledge of pure - blood wizarding traditions helps them in some situations, and Harry's sense of justice and fairness influences Draco. This shared adventure in the past cements their friendship when they return to their own time.
Perhaps it's a comical situation within the fanfic. Harry, being his curious self, accidentally licks Draco's wand while they are in a heated argument and it causes some sort of magical mayhem, like the wands start to emit strange lights and sounds.