Holding a gun makes you think others are too, new research shows

“Beliefs, expectations and emotions can all influence an observer’s ability to detect and to categorize objects as guns,” Brockmole says. “Now we know that a person’s ability to act in certain ways can bias their recognition of objects as well, and in dramatic ways. It seems that people have a hard time separating their thoughts about what they perceive and their thoughts about how they can or should act.”

Could be a good example of the theory behind the Florida shooting.
Florida teen's shooting by watchman questioned