It is moral outrage. People hate rats and other pests, and while the visceral emotion is disgust, the primordial need for self preservation is behind the mechanism that causes the reaction. After all, insects and rodents often carry infectious diseases and there are varieties of insects that are either poisonous or carry disease.

This is what's happening with mass shooters. There is fear and disgust, because there is a belief that it can happen to them. There is also moral outrage because it is so outside the bounds of what most people consider acceptable or rewarding behavior. After all, there are no rewards for killing masses of people unless you enjoy the power, control and satiation of sadistic impulses that such an act offers.

Not everyone finds that rewarding and even fewer people find the rewards to be worth the possible punishment. These people are sloppy psychopaths, because these drives can be satisfied over the long term without the jail time that is risked in the case of a mass shooting.