Speaking about Batman killing, I was reading Batman The Golden Age Volume 2 and on the third issue or so there's
Editor's Note: The Batman never carries or kills with a gun!
That is the earliest retcon I saw in the series I think because he shot The Monk and Dala with silver bullets and killed one of Dr. Strange's Monster Men with a gun from his plane only... half a year of publication ago at the farthest.
That note is after he got Robin and the books started to issue moral messages for kids.