The Boy in the Striped Pajamas Chapter 13

Bruno doesn't understand that he can't be beaten because he isn't a Jew, he doesn't understand how big Poland is, and he doesn't understand that there's no possible way that Pavel and Schmuel could know each other when there are thousands of people on his side of the fence. 

During dinner, Pavel was getting weaker and weaker and he went to open a bottle of wine for Pavel. He poured Bruno's father a glass and went to pour Kotler one when he spilled it. It landed on Kotler and Kotler beat him with everyone watching. 

I can infer that Kotler doesn't have much in common with his father and he's going against his father's wishes. I can also infer that Kotler will get into a lot of trouble because of his father. He chose to beat Pavel because he was angry at the way the dinner conversation had gone and wanted to take his anger out on someone he could actually hit.