Why did the Wallace family treat people the way they did?

Well, they treated blacks like garbage. I'm not sure how they treated their white friends. The attack on Papa by the Wallaces is a symptom of the violent racism the Wallaces possess. Why do they do this? I suppose that is one of the themes of the book and a question for the ages. What makes people like the Wallaces so full of hate and malice is a complicated question.