Unfamiliarity and thus fear of the unknown and the unusual.

Look at people's perception of the bombings of Hiroshima and Nagasaki. People are appalled, shocked, disheartened at the level of destruction, the inhumanity, the suddenness of mass death. Yet look at the perception of the fire bombing of Tokyo or the Rape of Nanking. Much less well known events in general and also typically events that don't stand out in most people's minds the way Hiroshima does. But these are all events of roughly comparable horror.

Nearly a hundred thousand individuals died in the firestorm that engulfed Tokyo after the bombing. Imagine the horror of dying by fire. Surrounded by flames. Trapped in a collapsed building unable to flee to safety. Slowly succumbing to smoke inhalation or feeling the flesh of your body slough off as fire consumes you and then eventually you pass out from shock or blood loss. It's horrific. It ought to keep people up at night.

And then consider Nanking. Nearly a quarter of a million civilians systematically tortured, raped, and slaughtered for no other reason than to instill terror and for sport. Mothers, wives, and daughters brutally raped by Japanese soldiers and then killed and their corpses mutilated by jamming bottles or sticks into their genitals. Women raped until they died. Men, boys, and babies used for bayonet practice. A race between two officers to see who could kill 100 civilians with their sword first. A pregnant woman raped and then her abdomen slashed open and the fetus stabbed because she resisted. These are mind rending horrors. They ought to stick in our minds. But they are in truth very comfortable horrors. Horrors we are familiar with, horrors of the sort that have been with us for centuries or indeed millenia. Does anyone believe that the mongol horde treated its conquered peoples any better than the Japanese did in Nanking?

Rape, murder, fire, swords, bullets. These have become familiar, we know how to contextualize and even rationalize such things, as necessary. It is only when we lose the ability to contextualize that we freak out.

For example, there is a whole class of social ills, crimes, and so forth which have existed for decades or centuries but once they are translated into a new realm they become suddenly new and newly frightening. When someone kills another person due to a fight over something petty such as a game of baseball, that's old hat. Or if someone murders another in order to steal their car, that's also not at all interesting or unusual or necessarily particularly frightening. But if a dispute over a video game results in homicide or if someone is murdered to facilitate theft of their collection of MtG cards then that is suddenly a brand new thing that is newly horrific.