Perhaps the assassin is being controlled by a powerful dark magic artifact. In the 'Harry Potter' universe, there are many such artifacts that can compel people to do evil deeds. The assassin might not even be fully aware of their actions. It could also be that Moody had discovered some dark secret that the assassin was involved in, and killing him was a way to keep that secret buried.
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.
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.
In a 'Harry Potter is an assassin' fanfic, the plot could start with Harry being trained by an unknown and powerful wizard who has his own motives for creating an assassin out of Harry. This trainer could be someone from a far - off land or an ancient order. Harry then starts taking on targets within the wizarding world that are supposedly 'evil' but as he delves deeper, he realizes that things are not as black and white as he was told. His friends start to notice his strange behavior and he has to either come clean or keep lying to them. As the story progresses, Harry might have to make some tough choices between his loyalty to his friends and his new - found assassin duties, all while uncovering secrets about the wizarding world that were previously hidden.
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.
One possible motive could be a severe misunderstanding. Maybe Harry is tricked into believing that Snape and Moody are involved in some dark plot against him or his friends. For example, if there were false evidence planted that made it seem like they were working for Voldemort again. Another reason could be that he was under the influence of a powerful dark magic that clouded his judgment and made him act out of character.
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 an international wizard who has been trained in both magic and martial arts in some far - flung magical school. This wizard - assassin has his own code of ethics and when he meets Harry, he decides to take him under his wing, believing that Harry can be a force for good in a different way than the traditional Hogwarts - trained wizards.
In the fanfic, perhaps Harry had reached his limit with Umbridge's blatant disregard for the rights and well - being of the students and magical creatures. She was a symbol of the corrupt Ministry at that time. Harry, having a strong sense of justice, could no longer tolerate her presence and actions, especially after seeing how she treated his friends and those he cared about at Hogwarts.
In a 'Harry Potter kills Lucius Malfoy' fanfiction, the motives could be complex. It could start with Lucius's long - standing enmity towards Harry and his family. Harry may have reached a breaking point after all the years of Lucius's attempts to undermine him. Additionally, in the context of the wizarding war, if Lucius is seen as a key figure in Voldemort's regime, Harry might view his death as a strategic move to weaken the Dark Lord's forces. It could also be a way for Harry to assert his own power and dominance over the Malfoy family, who have always looked down on him.
One possible motive could be a dark magic or mind - control situation. Maybe an enemy has the power to control Naruto's actions and forces him to kill Sasuke against his will. Another could be a miscommunication where Naruto believes Sasuke has done something unforgivable, like harming the village in a way that Naruto can't overlook.
Perhaps in the fanfic, Snape discovers a powerful and dangerous magic that he wants to use for what he believes is the greater good, but Harry sees it as a threat. Harry has always been wary of Snape, and this new discovery could be the tipping point. Also, if Snape were to make a deal with a dark force in order to gain knowledge to defeat Voldemort but Harry misinterprets his intentions, it could lead to Harry killing Snape.