One of the most interesting aspects of this classic movie is the way that Sam Spade thinks though the moral dilemmas. When he is deciding whether to tell the police about Brigid, he is very explicit about weighing every aspect of his choices. It is not an easy decision for him; he has no moral absolutes. On one hand, he loves her, and he didn't think much of his partner. On the other, he doesn't trust her, he doesn't think she trusts him, and he knows that they could not go on together, each waiting to betray or be betrayed. If he turns her over to the police, he loses her. But if he doesn't, he loses a part of himself, his own kind of integrity.
When this movie was made, moviegoers were used to cool, debonair detectives (like Philo Vance and Nick Charles, both played by William Powell), a sort of cross between Sherlock Holmes and Fred Astaire. But Sam Spade, created by Dashiell Hammett based on his experiences as a detective, was a modern day version of the cowboy, a loner with his own sense of honor.