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.
Another possibility is that it's a case of mistaken identity. The shooter might have been aiming for someone else who looks like Ezra. This could lead to a whole lot of confusion and twists in the story.
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.
Perhaps a new character who has a hidden agenda. Maybe someone who has a connection to a current case Gibbs is working on and doesn't want him to get too close to the truth. This person might be working for a larger criminal organization and sees Gibbs as a threat.
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.
Perhaps a mysterious stranger who has some unknown vendetta against the Heartland family. This person could have been lurking around the ranch for a while, waiting for the perfect opportunity to strike. Or it could be someone within the community who has a hidden agenda and sees Amy as an obstacle to their plans.
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.
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 Nicole's from her police work. Maybe someone she put in jail before who has a vendetta against her. This person might have been released recently and is out for revenge.
Maybe it's a jealous rival in the forensic science field. They see Brennan as a threat to their own reputation and success, so they arrange for her to be shot to get her out of the way.