None of the great monsters of the past hundred years -- Lenin, Mao, Stalin, Hitler, Pol Pot -- thought of themselves as evil. On the contrary, even when exhorting their followers to the worst extremes of human degradation, they did so in the name of a higher good. Hitler was purging the world of a dire threat, untermenschen (Jews, Gypsies, the mentally impaired, homosexuals, and Slavs), in order to usher in a golden age, the "thousand-year Reich." Even in the very last moments of his life, Hitler pointed to his war against the Jews as his greatest achievement. He was proud of the Holocaust.
The communists allowed as how in order to make an omelet, you had to break some eggs. In order to build the perfect society with universal prosperity and complete equality, some harsh measures would be necessary in the short term. But it was for a greater good. The brave dissident, Vladimir Bukovsky, noted mordantly that he had seen many broken eggs, but no one had ever tasted the omelet. One of Stalin's henchmen recorded, "Our great goal was the universal triumph of communism, and for the sake of that goal everything was permissible ... to destroy hundreds of thousands or even millions of people ... and to hesitate or doubt about all this was to give in to 'intellectual squeamishness' and 'stupid liberalism.'" One hundred million people in the 20th century were sacrificed to that particular ideal.
Robert Heinlein said that "Man is not a rational animal, he is a rationalizing animal." Some are. Certainly those who commit tremendous crimes nearly always do so armed with elaborate self-justifications.
The unifying theme for the great killers is to dehumanize their victims first. Stalin targeted "kulaks," and "counterrevolutionaries." Hitler despised "Jewish vermin." Pol Pot loathed and derided "cosmopolitans." The soft-spoken bin Laden, his quiet style notwithstanding, denied the humanity of his victims with the word "infidel."
You cannot reliably detect evil in a face. But the attempt to dehumanize is always the precursor of dark crimes.