Batman was perfectly willing to kill in his earliest appearances, as seen in his very first story, where he punches the villain into a vat of Hollywood Acid and shows no remorse for it. To be fair, it's not like Batman deliberately aimed to send him into the acid (a safety rail broke), but he sure didn't have any problem with what happened, declaring it a 'fitting end for his kind.'
In Detective Comics #32, Batman fashions a pair of silver bullets, draws a pistol, and shoots two dormant vampires, and #35 shows a title page image of him wielding a gun. While these are the only instances of him using a gun in the era, it's still rather bizarre in light of how the fact that Batman Doesn't Like Guns is now a defining part of who he is.
Readers weren't told about the death of Bruce Wayne's parents for the first six months of the character's adventures. These early adventures took place in New York City rather than the fictional Gotham. Also, he had purple gloves. And the Batmobile was originally a bright red sedan rather than a black car with a bat motif. In general, he was a lot less like Batman and a lot more like The Shadow (a few comics scholars have even suggested outright plagiarism), and didn't really become recognizably Batman in personality and operating methods until he got his own solo series.
Bruce Wayne originally had no servants in his home, unable to trust anyone with his secret identity (except Robin, once he showed up). When Alfred was introduced (four years after Batman's debut in Detective) he was a bumbling comic relief character who looked and acted almost completely different from the man we know.
A lot of Batman's Rogues Gallery were completely different in their initial appearances:
The Joker originally had No Sense of Humor (his name was instead more of a reference to his status as a Wild Card). His Silver Age self was closer to his typical depiction but was simply an affable, legitimately funny thief who liked to make people laugh and viewed Batman as a Worthy Opponent; a far cry from the creepy, murderous psychopath he would become famous as being. He would even sometimes team up with the heroes to take down worse villains, such as a memorable issue where circumstances forced him and Superman to work together in order to save Perry White. His clown-like complexion also used to be actual makeup, which he removed to pose as a cop (a scene that was referenced in The Dark Knight). Later issues would reveal that his complexion was a deformity from falling into a vat of chemicals.
The Scarecrow was originally a standard hoodlum-for-hire (albeit one who used to be a college professor) who terrorized his victims the old-fashioned way: with guns and death threats (in this era, fear gas was actually the gimmick of the now-comparatively obscure Hugo Strange). He also managed to hold his own against Batman and Robin physically, at least for a little while.
Clayface used to be a simple thief/murderer without superpowers who wore an old costume. He only got the upgrade to superpowers after it became clear that Batman and company completely outclassed him.
Mr. Freeze, one of the most famous examples of the Tragic Villain, was originally just your typical bank-robbing supercrook who cracked jokes and used cool Sci-Fi weaponry to pull off heists. Also his condition didn't change his physical appearance, he called himself "Mr. Zero", his freeze-gun couldn't kill people and, perhaps most shockingly, his first appearance actually ended with his body returning to normal after being exposed to steam. It took another nine years for the character to show up again, and even that was only because of his appearances on the popular live-action Batman show. After that, he mostly bounced around the various Batman books as a minor foe, before he was straight up killed off during the 1991 Robin mini-series. It wasn't until Batman: The Animated Series revamped the character that he was brought back in the comics, now reimagined as the pitiable and tragic character he's famous as.
Poison Ivy's debut contained none of the Gaia's Vengeance or Straw Feminist themes the character would later become famous for; her association with plants only went gimmick-deep, and even then it was largely metaphorical (she got into crime because she was a huge Attention Whore, who sought to stick in the public's mind like a case of poison ivy). She was also far more obsessed with material possessions like jewelry and clothing, with one Suicide Squad issue having her outright claim to hate the jungle after getting a tear in her tights.
Catwoman did not wear a costume at all, instead using disguises and trickery in her heists. She didn't even call herself Catwoman originally, instead going by the shorter alias "The Cat"; "Cat-Woman" was just a nickname. She was also more overtly villainous, in contrast to the Anti-Hero/Anti-Villain characterization most modern fans are familiar with.
As the most "epic" villain in Batman's rogues gallery, Ra's al Ghul is all but guaranteed to get away at the end of a story, whether blatantly or through a Never Found the BodyComic Book Death (which his loyal followers and/or daughter will no doubt undo with the Lazarus Pits). Which makes it kind of hilarious to reread his debut story, which ended with Batman getting a clean win over him and dragging him off to jail like any other crook. The Batman: The Animated Series two-part adaptation of thisstory, faithful in most other respects, conveniently left this part out.
While Damian would go on to be a well established animal lover while he was first infiltrating the batcave in Robin (1993) he complained about and hit several bats, saying he didn't know what his father saw in them. He was pretty stressed at the time and had just been injured when he didn't move his hand fast enough to avoid it being slammed in one of the caves emergency doors.
Comics-wise, the title Legends of the Dark Knight would be tales set during Batman's early years. However, only five issues would deviate from this ideal, all of them being multi-part stories and four of them being used for Knightfall.
The Kate Kane version of Batwoman was initially portrayed as a Lipstick Lesbian and implied to still be in the closet (or at least had been), in contrast to her depiction in Detective, where she sports a more masculine dress style and is established as having been open about being gay since her teen years (with her refusal to hide her sexuality being a major plot point in her backstory).
The magazine wasn't born as a Superman/Batman crossover series, but as an Anthology Comic, wherein Superman, Batman and other heroes like Zatara, Sandman, the Star-Spangled Kid, Green Arrow, Lando or Red, White and Blue lived solo adventures. Initially, Batman did not even have top-billing, his strip usually having the last slot. Superman and Batman's first joint adventure appeared thirteen years later in issue #71 (July, 1954).
Judging by the covers, the World's Finest were Superman, Batman and Robin, who was featured in all covers until issue #85 (December, 1956).
