Philosophers traditionally identify three kinds of evil: moral evil-suffering caused by the deliberate imposition of pain on sentient beings; natural evil-suffering caused by natural processes such as disease or natural disaster; and metaphysical evil-suffering caused by imperfections in the cosmos or by chance, such as a murderer going unpunished as a result of random imperfections in the court system. The use of the word evil to describe such disparate phenomena is a remnant of pre-Enlightenment thinking, which viewed suffereing (natural and metaphysical evil) as punishment for sin (moral evil). But at the trial of Adolf Eichmann, Hannah Arendt observed another kind of moral evil: men who comply, unthinkingly, with evil rulers, regulations, or unfair systems, perpetrating unspeakably cruel acts. In this "banal" form of evil, perpetrators shut off the knowledge that their victims are human beings.
-Terror in the Name of God by Jessica Stern pg.xxiii