I take it to be axiomatic that people are revolted by witnessing the shameless gratification of an appetite they do not share. - Quentin Crisp

I found that I had become so spinsterish that I was made neurotic not only by my life of domesticity but by the slightest derangement of my room. I would burst into a fit of weeping if the kettle was not facing due east.

It would be impossible to get through the kind of life that I have known without accumulating a vast unused stockpile of rage. Retaliation, though, was a luxury I could never afford. On the physical level I was too feeble. On any other I was not rich enough. I never dared to be rude to anyone. I never knew that I might not need him later. Long after fantasies of sexual excess had ceased to torment me, my imagination was inflamed by lurid day-dreams of having my revenge on the world.

Mass-murderers are simply people who have had ENOUGH.