Why was Elie angry with his father when Idek beat him in Night?
Idek, their "Kapo" or work commander, is a little off-balance mentally, and those who live and work in Elie's unit quickly learn to stay out of his way. In Chapter 4, prior to the scene where Idek beats Elie's dad, Idek has previously beaten Elie -- essentially for being in the wrong place at the wrong time. Thus, Elie knows how easily Idek can be provoked.
When his dad is, likewise, beaten, Elie is upset at his dad for failing to avoid Idek in the first place. Elie Wiesel (the author) likely included this passage to show just how much imprisonment had changed Elie and the others. As he writes, "I was angry with him, for not knowing how to avoid Idek's outbreak. This is what concentration camp life had made of me" (roughly the middle of Chapter 4; Page 52 in my version of the text).
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