Critics on left and right (the latter group mostly libertarians) think Americans get too worked up about terrorism. Your chances of dying in a terrorist attack are tiny, they say, well below one in a million, so there’s no reason to fear terrorism more than other sorts of crime.
Or, indeed, accidents. You can make just about any kind of sudden or violent death look insignificant by comparing it with motor-vehicle fatalities, which have been on a long decline but still run to about 30,000 per year in the U.S. While we should do what we can to reduce the total still further, the vast majority of us consider present levels to be tolerable. A strictly enforced national speed limit of 55 mph, for example, would cut traffic deaths even further, but we have decided that the costs and inconveniences of that draconian a measure would outweigh the lives saved.
Yet numbers do not tell the whole story, as examples from history and the present show. Surely no one would deny the horrific effectiveness of lynching in oppressing African Americans after the Civil War. Still, the total number of lynchings was about 4,000, and if you look at the period up to 1920 (after which lynchings declined sharply) and do the math, an African-American male’s chances (the victims were almost all male) of being lynched in a given year were roughly 1 in 50,000.
But that’s not the point; the point is that lynching was part of a comprehensive system of brutality and humiliation, and every African-American knew that if he was suspected of violating the code, he could be the next victim. For similar reasons, a handful of anti-gay murders (e.g. Harvey Milk, Matthew Shepard) have attained great importance because they highlight how gay or transgender people face the daily danger of being targeted for violent attacks. If you believe the questionable assertion that cops are racist and target African-Americans, the same reasoning would apply to police shootings. If someone is actively out to get you, instead of just random chance, you feel in much greater danger.
With firearms, the logic is different. Most gun-control advocates do not feel personally targeted by gun owners, but mass shootings, understandably, scare them in a way that individual ones do not. This is also why people are afraid of flying, even though it’s safer than driving; deaths of large groups of people inspire more fear than seemingly random individual ones. This may be less rational than the case where you are targeted, but it is a very powerful, visceral fear.
And that’s why terrorism is so uniquely worrisome: It has both these elements. The U.S. is the specific target of many terrorist groups, and terrorism has the ability to kill hundreds or thousands of people at a stroke (or millions, with an atomic bomb). If a cause of death strikes randomly and kills small numbers of people at a time, it’s seen as just a part of life; if it is targeted but you are not the target, it’s probably a low priority for you; if it kills rarely and randomly but does so in groups, you can still take shaky comfort in the odds. But if you are in the target group and it has the potential for mass casualties, it is quite legitimately something you should fear.