Would you say Noir genre evolved because of WW2?

Yes, and the events leading up to the war as well,--rise of Hitler in Germany and of fascism in general, refugees from Nazi persecution finding work in Hollywood--these and other factors were I'd say key elements in the rise of Noir. That the more or less first or (if you must) canonical noir picture, Stranger On The Third Floor, was released in 1940, is significant. That one of the leading players in that film, Peter Lorre, had played the child killer who was the focus of the manhunt in Fritz Lang's M, released eight years earlier, is worth mentioning, too.

Certainly many of the lead character in Noir movies were veterans, which figures since they were able bodied, younger men of action. Often there is a script reference to their service. It seems to me that an element of French existentialism is woven into the plots of Noir movies, perhaps unconsciously by the writer(s). That seems only right considering the impact of the war on so many. Then there is the movie "I Wake Up Screaming", 1941 and pre Pearl Harbor. Whether this crime drama can be rightfully considered Noir is debatable. I see it as a precursor.