What aspects of humanity are being critiqued? How ? and Why?
The Maltese Falcon by Dashiell Hammett is what is known as a "hard-boiled" detective story, in which the typical pattern of the genre of a lone detective solving a mystery is transposed to a gritty, urban setting. The novel is intended primarily as entertainment rather than social critique, but within the book are some implicit critiques of human character.
First, the book does not have an heroic or unambiguously good figure. Unlike more traditional detective heroes, Spade himself is morally ambiguous, having an adulterous affair with Archer's wife and primarily focused on earning his living. There are really no unambiguously morally good characters in the novel. Everyone lies, has secrets, and acts primarily from self-interest rather than a desire for the common good. Even what appears as love in the novel is revealed as lust. Nonetheless, Spade does have a code of honor that causes him to act justly at the end of the novel, cleaving to that as a way to make meaning in a random world.