Google defines hero as "a person, typically a man, who is admired or idealized for courage, outstanding achievements, or noble qualities" and/or "a submarine sandwich".
Since Double Indemnity has a serious lack of sandwiches, the first definition is the working definition for this post.
No one is a hero forever. At different parts in life, people make mistakes and do great acts of kindness. The film is no anomaly to this. A hero, by definition, must be "admired" or "idealized". Whether the person is fit for admiration, lies in the eye of the beholder.
Walter Neff is a hero to his boss towards the beginning of the movie. The boss compliments and praises Walter when suggesting that he be promoted. The average person would not call a simple insurance salesman a hero, but by this definition and his boss' display, Walter is a hero. His "outstanding achievements" and "noble qualities'' to his specific job are why his boss offered him the promotion in the first place. During this scene, Walter is a hero to the boss no matter what the audience or other characters choose to believe.
Walter is also a hero to his love interest, Phyllis Dietrichson. While plotting the murder and committing the murder, he represents a man "idealized for courage" to Phyllis. Even though she is the true mastermind using him, there is a place in her where she acknowledges his courage to go through with it. Walter does not resemble the popular idea of a hero by murdering Phyllis' husband, but he is still a hero in someone's eyes.
A hero is not a person who is admired by most, nor is it some fantastical person who represents the absolute good of mankind. Nobleness, achievements, and courage live in a gray zone that each person must decide which is which for his or her own self. When Double Indemnity is called a "movie without a hero", the person is taking great liberties in saying his or her way of thinking and defining a hero is the true, absolute way. Arguments could be furthered in saying that some people value the certain traits that Walter and Phyllis upheld, but for the sake of argument, the common definition most agreed on was used.
