Well, in some versions, Fred might use his quick-witted magic skills. He could cast a distraction spell to draw Bellatrix's attention away from Hermione. Maybe a spell that creates multiple illusions of himself, confusing Bellatrix long enough for him to grab Hermione and disapparate them to safety.
In some fanfics, Harry might be able to tell Fred and George apart by their different interests within the story. For example, Fred could be more interested in potion - making experiments that are a bit on the wild side, while George is more into the business side of their joke shop plans. So when they talk about their hobbies or future plans, Harry can distinguish between them. Also, their sense of humor might be slightly different. Fred might crack a certain type of joke more often, like those related to magical creatures in a really absurd way, while George prefers jokes about Hogwarts professors. Through these nuances in their personalities, Harry is able to tell them apart.
In fanfiction, Fred's worry about Hermione breaking her arm can be a catalyst for character growth. His worry forces him to face his feelings for Hermione. This internal struggle can add depth to the plot. He may also become more assertive in trying to keep Hermione safe, which could lead to conflicts with other characters who have different ideas about how to handle the situation. This can create a complex web of relationships and events that drive the story forward.
There could be a symbolic relation. 'Pulp Fiction' is filled with themes of fate, redemption, and violence. Fred's 'dead' state could be a symbol of the ultimate consequence of the actions and choices that characters make in that seedy underworld. For instance, the movie shows how small decisions can lead to big consequences, and Fred being dead could be the end result of a chain of such events. It could also be that Fred represents a certain type of character who is doomed from the start in the 'Pulp Fiction' - like world, and his death is a statement about the nature of that world.
It can create a lot of tension. For example, if the OC wants to explore a forbidden area of Hogwarts for some important reason, like finding a hidden magical object, Fred's overprotectiveness would stop them. This leads to a conflict between the OC's goals and Fred's need to keep them safe.