An interesting theory. Perhaps as a reaction against the skiffy Batman of the fifties and the Camp Crusader of the television series, it may have been the case that the tone of the series steadily began to darken even before Denny O'Neil and Neil Adams returned the character to basic first principles in the seventies. Look at some of these sixties Batman titles and artwork, for instance:


www.superworldcomics.com/blog/batman-in-the-60s/