One possible reason could be a dark magic curse. In many fanfictions, an ancient or powerful dark magic spell might be placed on Harry, and despite all his skills and the help of his friends, he couldn't overcome it. This could be something that was secretly cast on him during his early years, perhaps related to his connection with Voldemort.
Ron would be full of anger and sorrow. He might direct his anger at himself, thinking he could have done more to protect Harry. He would probably become more reckless in his actions, trying to avenge Harry's death. His relationship with Hermione might also be strained as they both deal with their grief in different ways.
Yes, there are such fanfictions. Some authors like to explore the 'what if' scenarios where Harry's character takes a darker turn. It could be due to different upbringings in the fanfiction world, like being more influenced by the dark side of magic or having a different set of experiences that lead him to embrace darker magic from an early age.
One reason could be a dangerous magical accident. Maybe he was experimenting with some powerful and unknown magic that went wrong. Another could be due to a dark wizard's plot. The dark wizard might have found a way to bypass all his protections and kill him.
Well, perhaps they could become friends quickly. Bill might introduce Harry to some interesting magical things he hasn't seen before, like some unique creatures. And they could start having little adventures together around the wizarding world.
Harry might react with fear. He could be traumatized by the experience and be constantly on edge, jumping at the slightest sound. This fear could make him more cautious in general.
For Hermione, it would be a huge blow. She would likely feel a great sense of guilt, thinking she could have done more to prevent it. She might become more withdrawn and throw herself into her studies even more, but also be haunted by memories of Harry. Ron would be devastated too. He might lose his sense of humor for a while and become more serious. He would also be very angry, perhaps vowing revenge if Harry was killed by someone.
One potential consequence could be early detection by the Ministry of Magic. In the original series, under - age magic is closely monitored. So in fanfiction, this might lead to Harry getting into trouble with the authorities much sooner. He could face warnings or even expulsion threats from Hogwarts if the school finds out.
One reason could be a different family situation. For instance, if he was raised by someone other than the Dursleys, who might have been more involved in the dark arts. Another could be his experiences at Hogwarts. If he faced more discrimination or was wrongly accused often, it could lead to a sense of bitterness and a turn towards the dark side.
One possible plot development could be that Harry develops a fear of dogs. This fear might then become a character trait that affects his actions in other situations. For example, he might be hesitant to enter certain areas where he suspects dogs could be present. Another development could be that the attack was not accidental. Maybe there's a dark wizard using the dog as a tool to harm Harry, and this discovery could lead to a whole mystery-solving adventure for Harry and his friends.