DC Comics laid the groundwork for superheroes as fans know them, but a lot has changed since the 1930s. One big thing is the no-kill rule the company instituted in 1940. Superheroes in the first two years of the Golden Age of comics were more blasé about killing but they quickly pivoted to a younger audience, introducing characters like Robin and converting gun-toting heroes like Batman into non-lethal champions of justice. While things have changed in the modern era, many of DC's best heroes still won't kill their foes.
Source：https://www.cbr.com/why-dc-comics-superheroes-superman-batman-wont-kill/
