An antihero is a protagonist who lacks the usual noble qualities one might find in a traditional heroic figure. This type is common to film noir, a genre that presents the world as a dark and often nihilistic place. Most of these anti-heroes are dupes victimized by chance and driven to criminal behavior, but some are more proactive, the most famous being the hardboiled detective most famously personified by Raymond Chandler's Philip Marlowe.

Sam Spade is a classic example of this kind of noir antihero, predating Marlowe by several years. The book describes him as a "blonde Satan," suggesting his amoral qualities right away. In the most famous film version, Humphrey Bogart's Sam is not blonde nor particularly satanic-looking, but he retains his ambiguous behavior.

Sam is not the usual noble crusader who is morally spotless in every area of his life. For example, he has no affection for his partner Miles Archer, to the point where he is willing to sleep with Archer's beautiful wife without much remorse. However, he is professional enough to want Archer's killer brought to justice, even when he discovers that said killer is his newest lover, Brigid O'Shaughnessy. With the juxtaposition of his moral code and willingness to get violent, he exists between the shady criminal underworld and the legitimized world of the law, making him a true noir antihero.