The Kapos, who were also prison guards and sometimes Jews themselves, did whatever the Nazis asked of them. They were usually employed to watch over the barracks and supervise the work that the other prisoners did. If the prisoners worked too slow or too fast or did anything that a Kapo or guard did not like then the Kapo would use whatever force they felt necessary. Kapos like Idek would find reasons to beat up Eli and his father and certainly took advantage of his position of power over the other prisoners.
The Kapos were not the only prisoners who were employed to work in the camps. Soderkammandos were also prisoners who were hired by the Nazis to work int he camps. Their job was to prepare other prisoners for the gas chambers. They would take teeth, hair, personal belongings, and deliver the speeches that the prisoners were only headed toward the showers.