What is the main conflict in The Boy in the Striped Pyjamas?

The main conflict in the novel, The Boy in the Striped Pajamas, involves Bruno and his relationship with Shmuel. Bruno, unaware of the differences between them or the reason why Shmuel is kept on the other side of the fence, doesn't understand and is afraid of the way Shmuel is treated by others. When Kotler abuses Shmuel, for example, Bruno stays quiet rather then stick up for his friend. Bruno is aware that he didn't respond like a true friend and promises Shmuel he won't leave him. He proves this in the end of their story by staying by Shmuel's side, as he unknowingly walks to his death.