In fact, police don't usually need much incentive to shoot and kill members of the public.
Police have shot and killed Americans of all ages—many of them unarmed—for standing a certain way, or moving a certain way, or holding something—anything—that police could misinterpret to be a gun, or igniting some trigger-centric fear in a police officer's mind that has nothing to do with an actual threat to their safety.
So when police in Florida had to deal with a 19-year-old embarking on a shooting rampage inside Marjory Stoneman Douglas High School in Parkland, Fla., what did they do?
There were four armed police officers, including one cop who was assigned to the school as a resource officer, on campus during that shooting. All four cops stayed outside the school with their weapons drawn (three of them hid behind their police cars).