The original Batman was meant to be a dark and menacing character. He killed criminals, casually gunning them down or breaking their necks without remorse. Initially he even had problems with the police who viewed him as a nuisance.
Humor was not a major element. The appearance of such can be attributed from today's perspective. What was intended to be menacing in the late 30's/early 40's can easily be construed as campy/humorous today. The decision to include lighter fare later on was made to make the character more relateable to the it's audience of children (good relations with the police, Robin, more comic relief).
Bob Kane and Bill Finger stylized Batman on the old pulp magazines that were grim and lacking in humor. When the became clear that the character was a success, it underwent certain changes to maximize popularity. It's documented that Kane regretted the disappearance of the solitary and sinister Batman of the early days.
No way did Batman start out light-hearted, nor should he be portrayed that way.
edit: I wrote this awhile ago and now see that it wasn't necessary. Damn delayed loading.