Was the Batman back story not always what it is now? "Dance with the devil in the pale moonlight" and such.
The early Batman comics were kind of a masked Phillip Marlowe type of noir, but simplified for kids.
Robin and the Comics Code made Batman into the gimmick and lighthearted stuff, and the Joker as kind of a guy who hit you in the face with a pie rather than kill you guy.
The Joker didn't have a name or backstory, which was what made him unique. It also wasn't explained whether his skin was white and hair green, or if he wore make up.
The move of Batman into a dark character started in the Frank Miller series, The Dark Knight Returns.That was in the early eighties. The Keaton movie was the one that put Batman in all all black suit, and a lot of the feel of the movie was lifted from the Dark Knight, plus the weirdness of Tim Burton as a director.
Not to create a slide, but Batman was originally a gun wielding vigilante not afraid of killing when he first came out in the 30s/40s. Then they backed off that pretty fast especially when the comic code thing came out in the 50s. late 50s to late 60s were camp. late 60s to early 80s was much more intellectual detective and then Frank Miller started the dark trend in the mid 80s. It’s oscillated a bit over the last 40 years, but much darker then the 50s-early 80s on average.
At least as far as the comics go.