Heroes in Double Indemnity?

An American film noir published in 1944 and directed by Billy Wilder, Double Indemnity, features a car insurance salesman being conned by a beautiful woman to murder her husband.  The insurance claim placed on the husband makes people working in the insurance company wary of this event, and some are suspicious of the wife murdering her husband along with an accomplice.  Because of the film genre, or because I'm not just looking deep enough into the characters and their motivations, I cannot find what I consider to be a hero in this story, except for the man who reports talking to Mr. Dietrichson (Jackson).  In order to explain my thoughts thoroughly, I will pull definitions some definitions into consideration.

Is our protagonist really a hero?

After a quick Google search of the phrase "definition of hero" these are the three primary definitions I found:

a person, typically a man, who is admired or idealized for courage, outstanding achievements, or noble qualities.

This is the definition I agree with the most, as it addresses the idea that anyone could potentially be a hero.  Under this definition, our protagonist Walter Neff, began as a hero because he resisted Mrs. Dietrichson's temptations, but ended as a non-hero because he fell under her charm.  In addition, he ended as a non-hero because he murdered somebody (two people!), which is not considered to be a very noble thing to do.  Keyes stands out as a hero under this definition because he perseveres through a lot to find out what actually happened to Mr. Dietrichson.  Another reason I agree with this definition is that it immediately rules out Mrs. Dietrichson (very manipulative), Neff's boss Mr. Norton (arrogant and choosing to be ignorant of the facts Keyes presented), Lola Dietrichson (helpless), the Dietrichson maid (annoying), and Zachetti (violent).  The only character I found with completely noble actions is Mr. Jackson, and that is why he is my only hero.  He reported what he saw because that is what he interpreted as the right thing to do.

the chief male character in a book, play, or movie, who is typically identified with good qualities, and with whom the reader is expected to sympathize.

Under this definition, the only person who can be considered as the hero is Walter Neff.  Neff in the beginning of the film is seen as a nice and honest man, but he suppresses those qualities when he encounters Mrs. Dietrichson.  The audience is capable of sympathizing with him, but he still does commit horrendous deeds.  I do not agree with this definition, even though it does bring into play the motives behind Neff's actions which I find extremely valuable when interpreting this film.  

(in mythology and folklore) a person of superhuman qualities and often semidivine origin, in particular one of those whose exploits and dealings with the gods were the subject of ancient Greek myths and legends.

Do I have to talk about how I disagree with this definition in regard to this film?

In this film, all of the characters seem to be a bit twisted in a way, appear to be helpless, or are not a major part of this film.  Not including any obvious heroes into this film established this story as more dark.  With all of this darkness surrounding the story line, there was no character that gave me (a part of the audience) a sense of relief, as I always felt tension.  If the goal of the film was to create tension and reflection into the lives of the audience (for which of us are truly heroes?), then the director and writers succeeded greatly.