Maybe it's a character who has a hidden identity. It could be someone who was thought to be dead but actually survived and is now seeking revenge. This person could have a grudge against Moody for something that happened in the past. Or it could be a new character created specifically for the fanfic, a dark wizard from a far - off land who has been hired to kill Moody.
Well, one possible motive could be that the assassin is working for Voldemort. Moody was a powerful Auror and a threat to Voldemort's plans. By eliminating him, it would clear the way for the Dark Lord's operations. Another motive might be personal revenge. Maybe Moody had put someone close to the assassin in Azkaban in the past, and this was a way to get back at him.
I'm not sure of a specific one off the top of my head. But you could try searching on fanfic websites like FanFiction.net or Archive of Our Own. Use the keywords 'Harry Potter', 'assassin', 'betrayed' and 'Moody' in the search bar to narrow down the results.
Perhaps the assassin is a double agent, and Moody's betrayal is part of a much larger scheme. The assassin could be pretending to be betrayed to gain the trust of the real villains. So, the assassin then uses this new - found trust to gather information and save the day in the end. Another plot twist could be that Moody is not the real Moody, but an impostor who betrays the assassin for reasons related to the impostor's true identity and motives.
I'm not sure if there is such a specific 'Harry Potter Kills with a Knife' fanfic. There are countless fanfics out there, and some might have very unusual or dark concepts like this, but it's not a common or well - known one in the mainstream Harry Potter fanfic world.
It could be that the assassin was initially a victim of some dark wizard's experiment. Harry, being the hero he is, feels responsible for the child and decides to raise him. In the process of raising, he unknowingly nurtures the assassin-like qualities in the child because of the unique circumstances and challenges they face together.
It could potentially mean that in some sort of extreme or alternate take on the Harry Potter universe, the character Harry Potter is involved in actions that put an end to fan - created fictional works. Maybe it implies that his actions in a particular story disrupt the fanfic narrative in a very drastic way.
On the other hand, it could also start some interesting discussions within the fandom. It might make fans think about the darker aspects of the wizarding world that were not fully explored in the original books. For example, how much pressure can a young wizard like Harry actually bear? However, it also has the potential to make some fans feel uncomfortable or even depressed, especially those who are more attached to the positive and heroic image of Harry.
It would be a very dark and unorthodox take on the Harry Potter universe. Harry is not typically associated with using a knife as a weapon in the original series. So, it would likely involve a major deviation from his character as we know it.
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.