Kapo

A prisoner in a Nazi concentration camp who agreed to help the SS and command other prisoners in turn for lenience. The Kapos were despised by their fellow prisoners, many of whom believed that to be a Kapo was to turn on your own community and betray your people. The Kapos were also just as harsh in their treatment of prisoners as the guards and SS. During the Nuremberg trials, Francois Boix (a former prisoner in the Mauthausen concentration camp) said that most other prisoners shared his belief that "it was better to die like a man than live like a beast" and that they "preferred to be beaten up, and massacred if necessary, rather than become a Kapo."

After the war, Kapos were tried as war criminals, the same as the SS officers they served. There is an ethical debate on this raging to this day-- should the Kapos, who did what they did because the alternative was near-certain death, be punished the same as the SS officers who were cruel out of choice? Many of the surviving prisoners believe that their punishment was fair and just, stating that the Kapos took pleasure in being cruel and were just as sadistic as the Nazis.

The word "Kapo" is an abbreviation of "KAmpf POlizei," German for "combat police." (thanks to liveforever)

If you're interested in Kapos and how the surviving ones fared after the war, information on the internet is scarce, but the book Kapo is definitely worth a look.