Great quote via A Sullivan

"To do evil a human being must first of all believe that what he’s doing is good . . . Ideology—that is what gives evildoing its long-sought justification and gives the evildoer the necessary steadfastness and determination. That is the social theory which helps to make his acts seem good instead of bad in his own and others’ eyes, so that he won’t hear reproaches and curses but will receive praise and honors. That was how the agents of the Inquisition fortified their wills: by invoking Christianity; the conquerors of foreign lands, by extolling the grandeur of their Motherland; the colonizers, by civilization; the Nazis, by race, and the Jacobins (early and late), by equality, brotherhood, and the happiness of future generations. — Alexandr Solzhenitsyn, The Gulag Archipelago.

Link.