What could be the reasons for Harry being dark from a young age in fanfiction?

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 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.

In some fanfictions, it could be due to a misunderstanding. Maybe someone framed Harry for a serious rule violation that Gryffindor's house values highly, like cheating in an important magical exam. And the evidence against him seems so strong that the Gryffindor house decides to banish him.

In some fanfictions, the author might create a situation where Harry's actions lead to great harm to Gryffindor as a whole. For instance, if he makes a decision that causes Gryffindor to lose a significant amount of points in a crucial time, like right before a major competition. This could lead to his censure as the house would see him as a liability. Also, if he gets into a feud with important Gryffindor figures, like the prefects or the head of the house, it could result in his censure.

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.

Another possibility is that he discovers some dark secrets about his family or his past. These secrets could shatter his perception of himself and his loved ones. It could make him question everything he has known and believed in. For instance, if he finds out that his father was involved in some immoral deeds in the past, it would be a huge blow to him and result in his sadness.

In some fanfictions, Harry might be expelled for being wrongly accused of using dark magic. Maybe someone framed him by planting dark magic items in his dorm. Another reason could be that he continuously breaks school rules in a way that the school can't tolerate, like sneaking into the Forbidden Forest too many times without permission.

One possible reason could be that he's wrongly accused of a serious crime, like using dark magic. In the wizarding world, such accusations can lead to expulsion. Another reason might be that he's framed by a powerful enemy who wants to get him out of Hogwarts. For example, a Death Eater could plant false evidence against him.

In some fanfictions, it could be a political move within Hogwarts. There might be a group that wants to undermine Dumbledore's influence, and since Harry is so closely associated with him, they target Harry by getting him banished from Gryffindor. This could also be related to the discovery of some long - hidden secret about Harry's family that makes the Gryffindors view him differently.

Another reason could be related to a new prophecy. Suppose a new prophecy emerged that said Harry Potter's presence in Gryffindor would bring great danger to the house. The Gryffindor students and faculty, believing in the importance of protecting the whole house, might have decided to banish him. But of course, this would be a very difficult decision as Harry has been an important part of Gryffindor before. And in the end, the prophecy might turn out to be misinterpreted.