It could be an ex - boyfriend who is jealous of Annie's new relationships. Maybe he couldn't handle seeing her move on.
It could be an old enemy of Chuck's. Maybe someone from his past who has a vendetta against him for something he did before.
It could be an old enemy of Ezra. Maybe someone from his past who has a grudge against him for some reason, like a business deal gone wrong or a personal feud.
Well, perhaps a new character introduced in the fanfiction. Maybe a spy from a rival agency who sees Abby as a threat to their mission. Or it could be an associate of a major criminal that the NCIS has been investigating. They might think that by shooting Abby, they can disrupt the investigation. Also, it's possible that it's a person with a personal vendetta against Abby, like someone she unknowingly wronged in the past.
Perhaps it was someone within Toby's own group who had a hidden agenda. A so - called friend who was jealous of Toby or had some ulterior motives that led them to arrange for the shooting. This would add a lot of drama and betrayal to the fanfiction story.
In a '90210 Annie Gets Shot' fanfiction, the plot could involve Annie's friends and family rallying around her. Maybe they try to find out who shot her and why. It could also show her struggle to recover in the hospital, both physically and mentally.
There's a possibility that it's a family member of a criminal she arrested. They might be seeking justice in their own twisted way. They could have been planning the attack for a long time, waiting for the perfect moment to strike when Burgess was most vulnerable. Another suspect could be a corrupt cop from another precinct who doesn't like the way Burgess and her team are cleaning up the streets and cutting into their illegal activities.
It could be someone from a past case she worked on. Maybe a criminal she put away who has a vendetta against her and managed to get out on parole early. They might see shooting her as a way to get revenge for the time they spent in jail because of her.
It could be a jealous rival. Maybe someone who is envious of Peter's success or his relationship with a certain person.
It could be an ex - boyfriend who was jealous and wanted revenge. Maybe he couldn't handle the break - up and decided to harm her in this extreme way.
There's also a possibility that it's an agent of a secret organization. This organization might be aiming to disrupt the hero society, and they see Bakugou as a key figure to target. So they send one of their best shooters to take him down.