It could also be a misunderstanding. Maybe Natsu is framed for something he didn't do, like stealing Lucy's money to buy food (even though he's just hungry and didn't actually steal). But Lucy believes it's true at first and starts to hate him until the truth comes out later in the fanfiction.
Perhaps in the fanfiction's plot, Natsu makes a big mistake that endangers Lucy or her friends. Let's say he recklessly takes on a powerful enemy without a proper plan, and Lucy's friends get hurt as a result. This might cause her to hate Natsu for his irresponsible behavior.
There might be a lot of bad writing in the fanfiction that involves Natsu. Poorly written dialogue, actions that don't make sense for his character, or just a general misrepresentation. If the fanfiction is full of these elements, it could give the impression that Natsu hates in that fictional world created by the writer, even though it's not true to his real character.
In fanfiction, Lucy might want Natsu because of their strong bond. They've been through many adventures together. Natsu is always there to protect her, and his brave and fun - loving nature attracts Lucy.
In a fanfiction, there could be a situation where Lucy is forced to side with a group that Natsu is against. This would make Natsu see her as an enemy. Or perhaps there is a prophecy that Lucy is involved in that Natsu misinterprets as something bad. He might think she is a threat to the guild or his friends. Also, if there is a love triangle situation where Lucy chooses someone else over Natsu in a way that really hurts his feelings, that could be a reason for him to start hating her.
He could kidnap her because he believes she's in immediate danger. Natsu is very impulsive and might not have time to explain things properly, so he just takes her away to keep her safe.
In many fanfictions, Natsu protects Lucy by using his powerful fire magic against their enemies. For example, if they encounter a dark guild attacking Lucy, Natsu would jump in front of her and unleash a flurry of fire attacks like his 'Fire Dragon's Roar' to drive the enemies away.
In some fanfictions, Lucy might use her Celestial Spirit magic. For example, she could summon a powerful spirit like Leo to help fight off whatever is threatening Natsu. The spirit could provide a distraction or directly combat the enemy while Lucy tries to reach Natsu and get him to safety.
He might be protecting her. Natsu cares a great deal about Lucy. If the secret got out, it could put her in danger, either from outside threats or from within the guild if they misinterpret the situation.
Maybe Natsu hates Fairy Tail fanfiction because he doesn't like how some fanfictions distort the original story or the characters. In the original Fairy Tail, he has a clear personality and relationships, and fanfictions might change that in ways he wouldn't like.
Sure, there may exist such fanfictions. In the official 'Fairy Tail' story, Natsu and Lucy are very close. But fanfiction is a world of creativity and deviation from the original. A writer might decide to make Natsu hate Lucy due to some made - up reasons like a misunderstanding or a conflict of interests. This could lead to a whole new set of adventures or conflicts within the 'Fairy Tail' universe as rewritten by the fanfiction author.
Yes, there might be such fanfictions in the vast world of 'Fairy Tail' fan creations. However, these types of fanfictions can vary greatly in terms of their content and quality. Some may focus on the comical aspect of such a situation, like Natsu accidentally stumbling into Lucy's bathing area and the chaos that ensues. Others might use it as a way to develop their relationship in a more platonic or budding romantic way within the fan - created narrative.