Batman developed his unwillingness to kill very early in his career. I believe even before Robin came along.
In any case, there were indeed exceptions to what I said. But the vast majority of the comics of the Golden Age were very optimistic in their tone and the heroes were indeed less cynical than they became in the Silver Age -- Superman, Capt. Freedom, Commando Yank, American Crusader, Hawkman, Capt. America, etc.
Heroes like them were the status quo during the 1940s. Now, I'll grant you that the 1950s comics were indeed dumbed down and even more innocent that those of the 1940s. By the early '60s, Marvel started to really change the comics field with FF, Spidey and the rest, instilling more intricacy to the stories told.