One possibility could be that Harry is under some dark magic influence. Maybe he's been corrupted by a powerful and evil spell or artifact that makes him lose his sense of right and wrong, leading him to turn on everyone. Another aspect could be that he's been betrayed so deeply by those he trusted that he snaps and decides to take revenge on everyone, believing they are all against him.
In this fanfiction, it could start with a great tragedy that befalls Harry. For example, he might lose all his friends and loved ones due to a series of events that he blames on the wizarding world at large. This overwhelming grief and anger could drive him mad. He may start to see everyone as the enemy, and with his powerful magic skills, he decides to go on a killing spree. It could also be that there is a prophecy that he misinterprets which leads him to believe that killing everyone is his destiny.
Well, perhaps in this 'harry kills everyone fanfiction', there is an alternate reality where Harry has a different upbringing. He might grow up in a more hostile environment that warps his personality and values. He could be part of a group that has extreme views and they convince him that the only way to bring about change or gain power is to eliminate everyone who stands in his way.
In such a fanfiction, it could be that Harry Potter has been corrupted by a powerful and malevolent force. This force might have warped his sense of right and wrong. It could also be that the author wants to explore the extreme consequences of power and how it can turn a hero into a villain. For example, if Harry Potter was constantly taunted and mistreated by the wizarding community despite all he has done, he might reach a breaking point and lash out at everyone.
One possible plot could be that Naruto is somehow corrupted. Maybe he's been influenced by a powerful dark force or a powerful enemy has tricked him into believing that everyone is a threat to the world he wants to protect. So, he decides to take extreme measures and kill everyone to 'create peace' in his warped sense.
Well, it could be a dark AU (alternate universe) where Naruto is pushed to the brink. Maybe he discovers some great betrayal among his friends and village. So in his rage, he decides to take revenge and starts killing everyone. But this is a very extreme and uncharacteristic concept for the original Naruto character.
In some interpretations, it could be that Harry finally snaps after years of abuse from the Dursleys. Maybe they pushed him too far one day and in a fit of rage, he uses his magic to end their lives. It's a very dark and extreme take on the Harry Potter universe.
Yes, it is quite disturbing. Harry is typically seen as a hero, and the idea of him killing everyone goes against his fundamental character. It subverts the positive and heroic image that fans have of him, and it involves a lot of death and destruction which is generally not pleasant to think about.
Maybe the goblins in the fanfiction have turned evil and are causing great harm to the wizarding world. Harry, seeing no other option, decides to take matters into his own hands and kill them to protect others.
It could start with a build - up of small incidents. For example, the Dursleys constantly belittling him even after he has proven himself in the wizarding world. Then, one major event, like the Dursleys trying to sell his magic items or preventing him from going back to Hogwarts, causes Harry to snap. His killing of the Dursleys could then lead to a search for redemption or a fight against those who want to punish him unjustly, as he was pushed to his limits by their cruelty.
In this kind of fanfiction, perhaps Harry feels extremely lonely. He might start to doubt himself and his worth. Maybe he tries to find ways to get noticed but always fails. For example, he could be left out of group activities at Hogwarts, and his friends seem to not care about his presence anymore. This could lead him to explore new aspects of magic on his own, away from the usual crowd, and in the process, he discovers some hidden powers or secrets that eventually make others realize their mistake of ignoring him.
Well, it could start with Harry being wrongly accused of something major, which leads to everyone abandoning him. He then embarks on a solitary journey to find the truth and clear his name. During this time, he might encounter old enemies who see his vulnerability and try to take advantage of it. However, Harry's determination and the few skills he has managed to retain despite the abandonment help him fight back. He also has internal battles with feelings of loneliness, betrayal, and self - doubt, but ultimately he rises above it all to show that he is still a force to be reckoned with.
Well, it could be a very dark and alternative take on the Harry Potter universe. Maybe it's a fan - created story where the character Harry, for some extreme reason like being corrupted or in a very different version of events, decides to kill everyone. But this is quite a departure from the original story's themes of friendship, love and fighting against evil in a more heroic way.