When the obviously disturbed John Hinckley shot Ronald Reagan in 1981, pundits blamed guns for the tragedy; they didn’t blame an ideology, left or right. In the New York Times, David Rosenbaum quickly dispatched with the theory that Hinckley harbored political motivations. Citing Lee Harvey Oswald and Sirhan B. Sirhan, Rosenbaum identified a line of assassins who “were deranged loners, social misfits apparently acting alone.” Hinckley, Rosenbaum added, “fits that mold.” Later, Rosenbaum enumerated three elements that “authorities agree contribute to assassinations”: lax security, a glorified presidency, and “availability of hand guns.” “Few doubt that it would be a deterrent...