I think it’s the forensic guy. He’s small. The person who shot the film makers was small (a woman or a small man). The two people who worked with Red John and later killed where in law enforcement (the deputy sheriff and the secretary). Remember when the secretary was killed Red John cut her with a poison. Someone no one would notice. Red John walk right past the guards and got to the secretary. Someone who could get strange poisons. Someone who knew how to shoot a gun.