"the line dividing good and evil cuts through the heart of every human being." But it can be crossed once and for all.

     "If only it were so simple!  If only there were evil people somewhere insiduously committing evil deeds, and it were necessary only to separate them from the rest of us and destroy them.  But the line dividing good and evil cuts through the heart of every human being."

     Aleksandr I. Solzhenitsyn, The gulag archipelago [(Архипелаг ГУЛАГ)] 1918-1956:  an experiment in literary investigation I-II, pt. 1, chap. 4, "The bluecaps," trans. Thomas P. Whitney (New York:  Harper & Row, 1973), 168.  On the other hand (174-175),

     There was a rumor going the rounds between 1918 and 1920 that the Petrograd Cheka, headed by Uritsky, and the Odessa Cheka, heading by Deich, did not shoot all those condemned to death but fed some of them alive to animals in the city zoos.  I do not know whether this is truth or calumny, or, if there were any such cases, how many there were.  But I wouldn't set out to look for proof, either.  Following the practice of the bluecaps, I would propose that they prove to us that this was impossible.  How else could they get food for the zoos in those famine years?...  Those enemies were going to die anyway, so why couldn't their deaths support the zoo economy?...  Wasn't it expedient?
     That is the precise line the Shakespearean evildoer could not cross.  But the evildoer with ideology does cross it, and his eyes remain dry and clear.
     Physics is aware of phenomena which occur only at threshold magnitudes, which do not exist at all until a certain threshold encoded by and known to nature has been crossed....
     Evidently evildoing also has a threshold magnitude.  Yes, a human being hesitates and bobs back and forth between good and evil all his life.  He slips, falls back, clambers up, repents, things begin to darken again.  But just so long as the threshold of evildoing is not crossed, the possibility of returning remains, and he himself is still within reach of our hope.  But when, through the density of evil actions, the result either of their own extreme degree or of the absoluteness of his power, he suddenly crosses that threshold, he has left humanity behind, and without, perhaps, the possibility of return.