In this novel Idek is the Kapo or commander of a group of prisoners who are charged with working in an electrical warehouse. Both the narrator and his father are in this Kommando or group. Before the narrator meets Idek, he is told by Juliek, another prisoner, that Idek has "occasional fits of madness" and is warned to try and stay away from him.
The narrator himself experiences one of these "fits of madness" one day:
One day when Idek was venting his fury, I happened to cross his path. He threw himself on me like a wild beast, beating me in the chest, on my head, throwing me to the ground and picking me up again, crushing me with ever more violent blows, until I was covered in blood. As I bit my lips in order not to howl with pain, he must have mistaken my silence for defiance and so he continued to hit me harder and harder.
Abruptly, he calmed down and sent me back to work as if nothing had happened. As if we had taken part in a game in which both roles were of equal importance.
Idek of course later beats the narrator's father, but then also arranges for all of his Kommando to work outside so he can sleep with a Polish prisoner. When the narrator sees him with the Polish girl, Idek later whips the narrator, threatening much worse if he ever speaks of what he saw.
Idek is one further example of how dehumanising the Jews makes them dehumanise other Jews and themselves. We see many examples of this throughout the novel, for example when a son willingly kills his father for a scrap of bread thrown to him by "helpful" German passers-by. Idek is given a limited amount of authority and he abuses that at whim to oppress others. Of course, whilst he is able to abuse his power in this way, he is still living under the same danger as his underlings, and so it is important to remember the same realities and threat of death would have affected him as well.