Answer:
In chapter thirteen of The Boy in the Striped Pajamas, Pavel—an old Jewish man and former doctor who is now working as a servant in the house of Bruno's father—mistakenly spills wine on Lieutenant Kotler's lap. The arrogant Kotler then drags Pavel into the next room and beats him so badly that he dies, leaving the maid, Maria, to clean up the mess.
This is startling to Bruno, who has two reactions to the beating. First, he decides (much like his father) not to get involved in such situations out of a desire to protect his own self from harm. Second, it forces him to start questioning what is occurring at Auschwitz. Bruno is generally willfully ignorant of his father's involvement in the atrocities, despite the fact that his promotion to Commandant has caused the family to move to "Out-With" (Auschwitz); however, he can clearly detect that something is not right after this incident.