Mother Jones has a useful graph:
So, the total number of fatalities from mass shootings since some point in the 1980s is roughly the same as the total number of homicides in Chicago this year (500).
On the other hand, most of the victims in Chicago are minorities. Most have criminal records. Indeed, a large fraction of the Chicago victims are drug dealers who might well have been the shooters themselves if only they'd shown more initiative (or better marksmanship).
Mass shooting victims, in contrast, are almost all random innocents. Indeed, the most recent mass shooters might be engaged in some kind of competition to come up with ever more outrageous samples of innocents, such as moviegoers or small white children.
By the way, you can see the collapse in mass shootings after the school shooting outside San Diego that I covered in early 2001. I argued at the time that we would see more and more such copycat crimes. Yet, the following year, 2002, was the only one with no victims since 1985.
Mass shootings are unlike most other kinds of violence in that there's no particular logic to their causes other than desire for notoriety and copycatism, both murky motivations. If the authorities let criminals, say, get away with carjackings, then there will be more carjackings. It's only rational. Yet, the rationality of these crimes makes deterrence more feasible. A variety of outrageous crimes — carjacking, kidnapping, home invasion, even mugging — have been increasingly deterred in recent decades.
But nobody ever gets away with a mass shooting.
Which means that, by definition, deterring mass shootings is difficult.
So, we need to prevent them. But the methods proposed for preventing such unusual and cloudy crimes — putting a brave marksman cop in every school, taking away 300 million guns, locking up the weird, arming kindergarten teachers, etc. — are not terribly confidence inspiring. Since nobody knows for sure what to do, this makes mass shootings ideal for arguing over. Everybody gets to expound on their favorite prejudice — stomp on rednecks, hire gun lovers to sit in schools, or whatever — without much connection to what will happen next.