The reaction would also depend on how the events leading up to these actions were presented in the fanfiction. If it was made clear that Harry was acting in self - defense or to protect the wizarding world from a hidden threat, some might come to his side. But overall, it would create a huge upheaval in the wizarding world. The Ministry of Magic would be under pressure to take action, and there would be a lot of unrest among the students at Hogwarts who looked up to both Snape and Moody in different ways.
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.
One possible motive could be that Snape is still deeply influenced by his loyalty to Voldemort in this fanfiction. Maybe Voldemort has some hold over him that forces him to do the unthinkable. Another reason could be a misunderstanding. Snape might believe that Harry has turned to the dark side or is about to, and he decides to take preemptive action to stop what he thinks will be a great evil.
Yes, it is a somewhat common theme in some fanfictions. One reason could be the complex relationship between Harry and Snape in the original series. Snape was often cruel to Harry, so in some alternate universes, writers explore the extreme scenario of Harry killing Snape. It also allows for exploring darker aspects of Harry's character and the consequences of such an act within the wizarding world.
One common theme is mistrust. Given their personalities in the original Harry Potter series, it's natural for fanfiction to explore how they initially don't trust each other. For example, Snape's double - agent status and Moody's cautious nature can lead to a lot of suspicion between them.
One possible motive could be a misunderstanding. Maybe Snape is framed for something really bad, like a dark magic plot against Hogwarts. Harry, in his haste to protect the school and his friends, believes Snape is guilty and takes drastic action. Another reason could be if Snape accidentally endangers someone Harry loves dearly, like Ginny or Ron, and Harry's emotions get the better of him in the heat of the moment.
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.
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.
One great Harry Moody fanfiction is 'The Adventures of Harry Moody'. It shows Moody in some unique and exciting situations that really bring out his character. The author does a great job of staying true to Moody's personality as we know it from the original series.
One popular Snape and Moody fanfiction is 'The Unlikely Alliance'. It explores their relationship in a post - war Hogwarts setting, showing how they have to work together despite their differences.