"Fate or some mysterious force can put the finger on you or me for no good reason at all."
That's the last line of “Detour,” a classic B-grade noir. It's also an expression of one of the defining characteristics of film noir, which is a pessimistic fatalism. Many noir movies feature some seemingly miniscule decision or act which sets in motion an inescapable chain of events and dooms the hero.
“DOA” is one of the best examples- the main character is a notary public, and he only finds out the reason for what happened to him at the very end, after it's already too late. All he had done was to notarize a document, an action taking only a few seconds. He didn't even remember doing it. Unfortunately for him, the document could have been used to expose a company smuggling radioactive materials, so he was marked for death.
Not every film noir has this characteristic, but many of them do. The sense of a dark and malignant fate, unstoppable and inescapable, gives noir a lot of its fascination. The noir hero is not heroic because he overcomes seemingly impossible odds, but because the odds really are impossible and cannot be overcome. We sympathize and identify with his doomed struggle to escape, but all along we know he won't.
Fate, in film noir, is an almost numinous power- a “mysterious force”- and this gives film noir a feeling almost similar to that of a horror movie. In fact, there is a certain amount of overlap between the two genres, and 1942s “Cat People” is sometimes referenced as both a horror film and a classic noir.