Well, perhaps in this fanfic, Harry has some special equipment that helps him kill the dragon. He could have found a magical sword that is especially effective against dragons, and with his courage and quick reflexes, he manages to slay the dragon. It might also be that he has the help of his friends in some way, like Hermione providing some crucial knowledge about the dragon's weakness.
Yes, there are likely fanfics out there with that plot. Fanfic writers often explore unique and creative scenarios like Harry killing a dragon. You can try searching on fanfic platforms such as Archive of Our Own or Fanfiction.net to find such stories.
In a more complex scenario, Bellatrix could be trying to use a dark magic ritual that would cause great harm to the wizarding world. Harry discovers her plan and has to engage her in a magical struggle. In the process, he manages to turn her own dark magic against her, which results in her death. This would show Harry's ingenuity and his ability to use the enemy's own weapons against them.
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.
One possible scenario could be that Naruto kills Kabuto in a fit of rage. Maybe Kabuto has just done something truly heinous, like harming one of Naruto's close friends. Another scenario is that it's a strategic kill. Naruto realizes that Kabuto is too dangerous to be left alive and makes a calculated decision to end his life. There could also be a scenario where Naruto is tricked into killing Kabuto by an enemy who manipulates the situation to make it look like Kabuto is a greater threat than he actually is.
One possible plot could be that Lily is under some dark magic influence which makes her turn against Harry. Maybe a powerful dark wizard has corrupted her mind, leading her to believe that Harry is a threat to the wizarding world that needs to be eliminated.
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.
In such a fanfic, one possible reason could be extreme depression. Maybe he faced overwhelming losses like the death of his closest friends and felt he couldn't go on. Another reason might be that he was under some kind of dark magic influence that made him lose his will to live.
Creative exploration. Some writers might want to see how the story would change if Harry had a darker side.
Dumbledore could potentially be a source of help. He is extremely powerful and wise. He could have given Harry some advice or a special magical item before the encounter with the dragon that would prove crucial in Harry's ability to kill the dragon.
One consequence could be that he becomes a hero among his peers. Everyone in Hogwarts might look up to him even more. The other students could see him as a real dragon - slayer, which gives him more popularity and respect.