Why do you think the did the author include the episode in which David had to work for the "evil-hearted" farmer?

This shows David's sense of duty and compassion, even to thse who ill-treat him. Narration reveals, however, that the man is in fact just a farmer, but a cruel and selfish one who lives with his equally cruel family on a remote farm. They take David in and, keeping him locked up in the barn at night, give him a bit of food in exchange for days full of hard work. But while he hates the farmer and his family, David realizes he never would have survived the winter on his own, and in fact has better food and better sleeping conditions than he had had in the concentration camp.