Double Indemnity is a conventional film noir in many ways- a femme fatale seduces the hero, an insurance agent, into murdering her husband in a complex insurance fraud. The main character Walter Neff is as intrigued by the prospect of doing a murder in this way, as he is by the girl. He wants to perform the act in such a way that his own company would be unable to detect it. Consequently the act is planned methodically and in such a way as to extract the most money from a Double Indemnity life insurance scheme. In a way Neff uses the murder not to say something to the Femme Fatale but to say something to his colleague- Barton Keyes- the chief investigator at the insurance company. The scheme breaks down because of the human failings of the femme fatale and of Neff- it breaks apart and leaves both of them spluttering out bloody orisons.
If the film is about anything it is about the relations between these three characters- Phyllis Deitrichson, Neff and Keyes. The murder in many ways is merely a symbolic thing that allows each of the three to exert power over the other one. Phyllis allures Walter into committing the murder. Walter convinces Phyllis about the way to do it and gives her the plan, he controls her movements and repeats his tense warnings to her constantly. Keyes of course is the inquisitor and Walter hopes to evade his investigation- by evading it in some way Walter will reveal himself as a more inspired investigator than Keyes, he will control him. In the end at the last moment as Walter breathes his life out upon the floor, Keyes is left standing the man in control. He even in a symbolic break with what has gone before lights Walter's cigarette.
In this sense the film is less about a murder- we see very little of the actual killing- we see less of the victim. All the other characters like Phyllis's step daughter are ciphers for the main three. The issue within the film though is how we define the relation of love- again the characters commodify love- they use it to exchange with each other. Keyes uses Walter's respect for him to try and tempt him to a new job under Keyes's direct supervision. Walter uses his professions of love to in some way justify himself to Keyes (remember that we are told the story through Walter's eyes), even at the end of his life as he tells the story he is trying to control our and Keyes's memory of the story. Phyllis uses love and desire both as a tool to get men into her pocket, greeting Walter in a towell, attracting him with a sexy anklet on her leg but also uses professions of love to create the duty of protection from others towards her. When Phyllis says I love you, what she really means is I need your sympathy and because I love you, you have to give me that sympathy. Love in the eyes of all three characters becomes a tool of power.
Post modernist Philosophers often talk about the relationship between truth and power- the relationship between the scientist and his subject, the observor and the object. This film in the characters of Walter, Phyllis and Barton Keyes brings that out again. Walter and Phyllis are separated by the murder- they can't see each other- so that after the murder what we have are three characters each moving through a solitary landscape trying to work out the motivations of the others. Keyes has a straightforward problem- to find the murderer- to find a matter of fact and he finds it. The other two though aren't trying to find out a fact, they know the facts, they are trying to find out each other's dispositions- they are trying to communicate because ultimately each wields the axe over the other, a confession and they are both dead. Hence it is that it is Keyes whose search is more dispassionate- despite the fact that he lives only for his work- rather than Walter or Phyllis who succeeds in his search for the truth.
The film is a meditation on all kinds of themes within human life- but perhaps at its most powerful it evokes Sartre. The ideas within Double Indemnity focus on the relationship between truth, love and power. The problem is that personal engagement, the film seems to say, brings with it a desire for love which brings with it a desire for power. The characters who get involved in life, become embroiled in struggle from which there is no exit- a struggle that ends with them dead. Keyes because he doesn't get involved, because he subsumes his own intellect and ego within the company, survives both spiritually (he doesn't commit a crime) and also actually- he is the one left alive at the end of the picture. The conclusion of the picture therefore is unremittingly depressing and ultimately too reductive- but it does present a picture of human relationships which is true to some degree. Love and murder smell like honeysuckle- power and affection are related- and in the mean streets of Film Noir their relation leads on to disaster.