Later in the novel, Huff comments that he never perceived his victim, Mr. Nirdlinger, as a real, living person, but rather as a figment of his imagination. (Cain also ensures that readers do not "know" Nirdlinger in the same way they understand the other characters, as he appears only twice in the story.) This statement highlights one of Cain's central themes, which is the hidden instinct within humans to kill without logical reasoning. Huff's motivations for murder include the irresistible allure of sexuality and greed, yet Cain demonstrates that an average working man, without obvious criminal inclinations, can be pushed to commit murder — a truly terrifying depiction of modern humanity in literature.

Another significant theme explored by Cain in Double Indemnity is guilt and its impact on the characters' psyches. Huff experiences physical illness following the murder, unable to shake off the haunting memory of his crime until he is caught. The overwhelming desire to escape the torment of guilt pushes him towards suicide with Phyllis, carrying with him in death the memory of their ruthless act.