In chapter 4 of "Night," Elie learns from Juliek, a fellow prisoner, that they will be working in an electrical materials warehouse. He tells Elie that the work isn't very hard but to watch out for "Idek, the Kapo, occasionally has fits of madness, and then you'd better stay out of his way." pg 50.
Elie learns first hand that what Juliek told him was true. One day while Elie was working Idek attacks him for no reason that he can think of. Idek beats Elie badly for several minutes then suddenly calms down and tells Elie to go back to work. (pg. 53)
Idek in this novel is a prisoner who is put in charge of a barracks. Thus he is given power over other Jews that we can see clearly he abuses and misuses. We are told that he is prone to violent rages, and when he is in one of his rages it is better to stay out of his way.
In Chapter 4, Idek savagely beats Eliezer, although he is unprovoked. Eliezer is given some comfort by a kind French girl who tends to his wounds. After a quick tangent in which the author mentions that he later met that French girl in his life, he returns to the second rage of Idek, who targets Eliezer's father this time. The narrator is savagely honest with himself, admitting that the camp has changed him so much that he cannot feel angry at Idek, but feels anger at his father because of his inability to avoid Idek's fury. We can see that the narrator has become obsessed with his own survival.
Lastly, Idek moves about a hundred prisoners to the warehouse so that he can have sex with a female prisoner. Eliezer witnesses this act, is discovered, and Idek whips him publicly until Eliezer faints.
The character of Idek then demonstrates the theme of insanity within the camps and also that of the abuse of power - he is given power and abuses it just in the same way that the Nazis abuse their power over the Jews. The abuse is replicated - and sadly, it appears that just because Idek is a Jew, it doesn't make any difference.