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 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.
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, in the 'NCIS Abby gets shot' fanfiction, she might get shot while trying to protect one of her colleagues. Abby is very loyal and would put herself in harm's way for her friends. It could also be that she is lured into a trap and then shot as part of a revenge plot against the NCIS team. The shot could be life - threatening, leading to a tense race against time to save her in the story.
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.
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 a former criminal that McGee helped put away. They might have escaped from prison and are seeking revenge on him for ruining their life.
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.
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.
A possible suspect could be an informant who has turned on Tony. Maybe the informant was promised a better deal by another party if they got rid of Tony. Also, it could be a new criminal on the scene who sees Tony as a threat. This new criminal might be trying to establish their power and reputation, and taking out a well - known NCIS agent like Tony would be a big statement. Another suspect could be a person with a grudge against the NCIS in general, and Tony just happens to be an easy target.