Perhaps a character with a hidden agenda. For example, someone who is working for an organization that wants to stop Blaine from achieving a certain goal. They shoot him to derail his plans and send a message to those associated with him.
It could be a random act of violence by a criminal who was in the wrong place at the wrong time. There are some bad elements in the area where Blaine was and they just targeted him without any specific reason other than opportunity.
A rival law enforcement agency member could be a suspect. Maybe they see Deeks as a threat to their own investigations or operations and decide to take him out. It could be a power play within the world of law enforcement.
It could be an old enemy from Beckett's past. Maybe someone they put in jail a long time ago, who has now been released and is seeking revenge.
Well, it could be a rival of Ciel's in business or in a social setting. Maybe someone who is jealous of his status or wealth and sees shooting him as a way to get ahead.
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.
It could be a case of mistaken identity. Sam might look similar to someone else who is involved in some sort of illegal or dangerous activity. The real target's enemies could have thought Sam was him and shot him instead. In a fanfiction world, this could lead to all sorts of twists and turns as the characters try to figure out who the real target was and why they were being targeted.
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.
Perhaps it's an insider within the law enforcement or criminal justice system who has a vendetta against the BAU. Maybe they disagreed with the way Derek and his team operate and decided to take matters into their own hands. It could be someone who feels that the BAU has too much power and is trying to send a message.
One possible plotline could be that Blaine's friends rush to his side after he gets shot. They then try to find out who the shooter is while also dealing with their own emotions. Maybe there's a mystery involved, like the shooting was a case of mistaken identity.
It could be that an anonymous assassin is the suspect. This assassin could have been hired by someone who wants Percy out of the way but doesn't want to get their hands dirty. Or perhaps a possessed character could be the suspect. They have no control over their actions when they kill Percy as they are being controlled by a malevolent spirit.
The butler could be a suspect. In many murder mysteries, the butler is often initially suspected. Or perhaps a jealous rival. If there's some sort of competition in the story, a rival might have a motive to commit murder.