Personally I don't think that Puzo didn't show reality. He always denied all charges of gloryfying them. And he did not. He's just objective. People are different, and who says that a gangster doesn't love his family? His novel is a great realistic work, he shows us what they did, and how, and what price they paid. Only he doesn't write didactical lessons, it is our business to derive them, and the absence of direct moralization is in my opinion a sign of good taste in literature.
As to bringing us to reality - when you see how the nice college boy you liked orders the strangling of his brother-in-law and surveys it calmly, don't tell me that nothing happens to your jaw!