An anonymous author wrote, "In literature, evil often triumphs, but never conquers." Any good story needs a conflict, and throughout history, the most basic literary struggle has been between forces of good and evil. Ultimate victory for the forces of good becomes that much more effective when it has had to overcome smaller victories for the opposing, evil forces. Evil may win the battle from time to time, but good will always win the war.
However, many works of literature present the ultimate triumph of evil over good. Just as in real life, the "good guys" don't always win; in fact, some of our greatest works of literature make far stronger and more effective thematic points by having the forces of evil win out over good in the end, even after leading the reader to believe that good will ultimately prevail. An author can use such a work as a cautionary tale, as a reflection of a grim or uncomfortable reality, or simply to surprise the audience.
Two such examples of this literary abnormality are Charles Dickens' classic novel, A Tale of Two Cities, and George Orwell's 1984.
In A Tale of Two Cities, set during the French Revolution, Charles Dickens depicts how pointless the rebellion becomes when the original goal of equality becomes lost when the anger, frustration, and desire for revenge of the third estate is finally discharged. Initially, the members of the upper, ruling class were the ones perceived as evil, maintaining their societal positions through a system of fear and repression. However, as the revolution progressed and power was shifted onto the lower classes, they too abused their upper hand, committing evilly heinous and merciless punishments and executions, illustrating yet another common theme of the corruption of an innately good individual once in power. Madame...