Well, perhaps he's under the influence of a powerful and evil spell. In the world of Fairy Tail, magic can often go awry. Natsu is a very passionate character, and if his emotions were manipulated by a spell, he could turn on his guild. Also, it could be related to some hidden past event that has suddenly resurfaced in his mind, making him think that Fairy Tail is to blame for something.
One possible motive could be that he's been mind - controlled. There are many villains in the Fairy Tail universe who have the ability to control others. Another reason might be a misunderstanding. Maybe he thought Fairy Tail had betrayed him in some way. It could also be a result of some dark magic that has affected his judgment.
The guild would likely be shocked at first. They consider Natsu as one of their own, so his attack would be completely unexpected. Some members might try to talk him down, like Lucy. She has a close bond with Natsu and would probably try to reason with him.
Erza would likely have a more serious take on it. If the fanfiction adheres to the values of Fairy Tail and portrays Natsu in a positive light that aligns with his character in the guild, she would probably approve. However, if it contains any inappropriate or un - Fairy Tail - like behavior, she would surely be against it.
It could be about Natsu having left Fairy Tail for some reason and then reuniting with the guild in a very unexpected way. Maybe he has new powers or a different outlook on life that causes some friction and also some amazing moments of reunion and growth within the guild.
Natsu could end up teaming up with some unexpected allies from Tartaros. Maybe there are some members who have a change of heart and decide to help Natsu fight against the more evil elements within their own organization.
She could be a key figure in an ancient prophecy. For example, the prophecy might state that only the son of a certain dragon - slayer mother (Natsu's mother) would be able to defeat a great evil that threatens the Fairy Tail world. This would set Natsu on a path to discover his mother's identity and fulfill the prophecy.
Well, one possibility is that Natsu could join Fairy Tail in a different way than in the original story. Maybe he has some special mission before officially becoming a member. For example, he has to prove his loyalty by helping an elder member with a personal problem.
One possibility could be that Natsu is attacked due to a misunderstanding within the Fairy Tail guild. Maybe a new member misinterprets Natsu's actions as a threat and attacks him in self - defense.
Natsu could also be deeply hurt. He values his friends highly in Fairy Tail. If betrayed, he might isolate himself, doubting his own judgment in trusting people. This could lead to a period of self - discovery where he tries to figure out how to trust again. For example, he might recall the times when his friends in Fairy Tail were truly there for him and use those memories to heal and grow stronger in the face of this betrayal.
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.
In such a fanfiction, Natsu might have become much stronger. Maybe he has mastered new fire - based magic techniques. He could also have new relationships within the guild, like closer friendships or even a romantic interest.