What is a Kapo?
The prisoners were organized into work units headed by a Kapo. The Kapo was also in charge of selecting the prisoners he worked with while at the camp. The Kapos were also prisoners, but they enjoyed preferential treatment due to the administrative roles they played. They were in direct contact with the SS officers, reporting the affairs of the other prisoners when requested. In order to secure their positions, some of the Kapos brutalized their fellow prisoners to impress the SS officers.
At Buna, Eliezer and his father were selected for the orchestra unit that also worked in an electrical warehouse. The unit was responsible for playing the military march for the other units as they went to work. Idek was the Kapo responsible for the unit working at the electrical warehouse, and he was known for his bouts of violence. In the story, both Eliezer and his father were unfortunate enough to experience the Kapo’s violent outbursts.
A Kapo was a concentration camp prisoner who cooperated with the Nazis to act as a police officer inside the camp. Even though they were prisoners themselves, they were given more food and were not tortured like the other people in the camps. So perhaps to keep their "privileged" positions, Kapos were notoriously brutal and often would beat the other prisoners for the smallest reason. Wiesel's father was beaten just because he asked permission to go to the toilet. Not all Kapos were evil, however. Wiesel tells of one who was executed because he had been hiding weapons for a prison resistance movement.
A kapo was a prisoner in the concentration camps who was given some menial administrative tasks. Some took advantage of their power and brutalized their fellow prisoners as badly as the Nazis did. However, some used their power to spare others or ease their suffering. Failure to do what the Nazis asked of them often resulted in their deaths.