How do you prove a negative? An armed civilian stops a gunman after he shoots one person. Had the armed civilian not been there, who is to say the gunman othewise would have shot 5, 10, 20, or more persons?
That is the devastating problem. There’s endless videos on liveleak.com and other sites of legal shots. They sometimes get the bad guy and sometimes don’t. But if someone is stopped it doesn’t make the news. Same as a plane or bus crash. It makes the headlines when it happens, not when something goes right.