The Question and Answer section for The Boy in the Striped Pajamas is a great resource to ask questions, find answers, and discuss the novel.
Kotler was indoctrinated into the Hitler Youth. He was brainwashed into believing Nazi ideology. Kotler also had little contact with family and few friends. He embraced Nazi culture as his identity.
I'm really not sure if Kotler was an athiest. It makes sense because Kotler only worshipped Hitler and the Nazi party.
Pavel had spilled some wine while serving it. Kotler was just full of rage. He hated Jews and felt he had a license to attack helpless people like Pavel.