There are many paths into darkness. This is only one way.
1. Language. How did Hitler manage to get an entire nation to acquiesce to the murder of millions of innocents? Through the manipulation of language. Calling political enemies and despised outsider elements in society parasites, rapists, murderers, lazy, or dangerous works a malignancy into the moral character of individuals. Repeat them often enough in different venues - fortunately Hitler didn't have the internet, he just had speeches, print media, radio broadcasts, records, and film - that was enough.
2. Passivity. Not to speak out, even a tiny bit, against a great, imminent evil. Not to say no when one's neighbors cry for violence against the innocent. Not to object when the neighbor's shop is vandalized. Not to help but to hide. From here, the steps into darkness are small and easy to make.
3. Action. Even small actions can bring an individual into darkness, can co-opt them. They can be brought over with one enabling act. It can be relatively innocent. Boycott a store because the owner is Jewish, Muslim, Hindu, hispanic, black, Irish, Asian, or any Other. Throw a rock through a window. Beware of those who encourage violence. Even just a shove and the perpetrator is on the long journey down into darkness.
That's all it takes. Even the best intentions, concern for the country's welfare, protection against a perceived threat, or devotion to an apparently harmless ideology, can be used to twist an individual.
There is a powerful moment in the documentary World at War, an interview with an elderly woman. Years ago during the war she was on a train in Germany. An air raid siren wailed. Everyone left the train. The woman was too tired to get off. She and a German soldier sat alone in their compartment. He started to talk. He told her about murdering Jews on the Eastern Front. How, before they mowed down the harmless farmers in an isolated village, the rabbi stepped up to them. "God sees this!" the rabbi said. It echoed in this soldier's mind until he lived in abhorrence of his own existence. He requested a transfer to a combat unit. He told the tired woman that he would seek death in battle. Don't be this guy. Don't travel into darkness. At some point in that dark journey, you, too, might wish for death to remove the stain.