'Double Indemnity' doesn't work

In 1944 it was nominated for 7 Oscars. The screenplay was written by Billy Wilder and Raymond Chandler. It is the first real Film Noir and it has an atmosphere thick enough to cut with a knife. And it ranks #29 on the American Film Institute's best American films of all time.

Still, it doesn't work on a character level.

Walter Neff (the story's narrator played by Fred MacMurray) confesses: “I killed him for money, and for a woman.” He sees the woman (Barbara Stanwyck) twice. He likes her a lot. The third time he sees her he agrees to plan and kill her husband so that she can get the insurance money. Perhaps after this they make love, we are not shown (we wouldn't be in 1944). Then he tells her that they must never meet again in private to avoid suspicion. He doesn't seem to want the woman. At no point does he ask for a cut, so he doesn't seem to want the money. Half an hour into the movie we know that he doesn't want the woman or the money. He is not shown to be a man of violent nature, more of a cold and calculating persona than anything else. Why then does he commit murder?

At the end of the film, Walter Neff goes to his insurance company office and confesses to a Dictaphone that he is the murderer (not the other person they suspect) and then attempts to run away to Mexico. Why confess when he is not suspected? Why doesn't he simply run away first, like any other murderer would? He commits murder for no reason then confesses and gives himself up. How can a man be his company's best salesman, plan and execute an intelligent murder and be so blazing stupid? Could it be that he is following the plot? Could it be that he is not a real person but a plot device?

In his 1944 essay The Simple Art of Murder, Raymond Chandler claims that murder doesn't belong to the British upper class and Agatha Christie but in the streets with Dashiell Hammett. He argues his points with conviction and we believe him. Then, in the same year, he teams up with Billy Wilder and gives us Walter Neff, a pulp character who can only live inside a Film Noir!