In Night, often animal imagery is used to describe the Jews--as in, they were herded or in a frenzy for food. In the passages where Idek the Kapo, their guard, goes crazy, Wiesel uses animal imagery to describe him, as well. He's clearly not a stable person, and he proves it by several incidents.
The first time Idek loses his mind, so to speak, is just after he has taken the men and boys in his unit to the electrical warehouse to do some rather light work. Out of nowhere, Elie says, Idek lost it.
One day when Idek was seized with one of his fits of frenzy, I got in his way. He leapt on me like a wild animal, hitting me in the chest, on the head, throwing me down and pulling me up again, his blows growing more and more violent, until I was covered with blood. As I was biting my lips to stop myself from screaming with pain, he must have taken my silence for defiance, for he went on hitting me even harder. Suddenly he calmed down. As if nothing had happened, he sent me back to work. It was as though we had been taking part together in some game where we each had our role to play. (p. 50)
The incidents get worse, as do the punishments, though there is at least some minimal reasoning behind the Idek's outrageous and cruel actions. While the other incidents are certainly more cruel, this, to me, is the most indicative of madness--it was random brutality without provocation.