Dashiell Hammett had a lot of worldly experience. He had seen all kinds of people and had a pretty cynical attitude about human nature. His novel is realistic in the sense that his characters behave the way many people do in real life. Many people care nothing about morality and care everything about money. Spade is untrustworthy because he has learned that to be trusting is probably the worst mistake you can make in this life. He doesn't trust Brigid, but Brigid doesn't trust him either. She has done nothing but cause him trouble since she first came to his office. She would have tried to kill him that same night if Miles Archer hadn't volunteered to follow her and Thursby instead of Spade. The police and the D.A. are trying to pin a murder on Spade. He is trying to save his life. He can only do that by finding out who killed Thursby, Jacobi, and his partner Miles Archer. They already have a good circumstantial case against him because he was having an affair with Archer's wife and because he is involved with all the people responsible for killing Thursby, Jacobi and Archer. These would be Brigid, Gutman, and Wilmer Cook.