Batman's refusal to kill the villains is a major theme in Batman comics, television, etc. Why?
Short answer. Batman is a fictional character. He does not kill his wonderful villains because the creators want to use them in future stories. Batman has the best rogues gallery of all the superheros. His villains are a major aspect of his popularity.
Let me note that Batman's villains are very bad, a bunch of sociopaths, in their fictional universe, but they are treasures of entertainment in our world. Such treasures should not be wasted.
Batman is particularly famous for his aversion to killing his villains, but other superheroes follow much the same code for the same reason.
Before proceeding let me address the typical historical explanation given on the Internet. Superheroes, particularly Batman, came out as darker characters. They killed, but mothers objected. So they cleaned it up for the kids. Fine, but it has been many decades since comics were a children's medium that worried about mother's opinions. The other rules that used to make the medium mom friendly were thrown out. The rule on killing was kept. Superheros do not waste, as in kill, villains because writers do not want to waste good villains.
The above paragraphs give us the short answer, but let us dig a little deeper. Why are superheroes so concerned with preserving their rogues galleries but other fictional heroes are not?
To answer this question it is useful to understand a bit about fiction. The story teller must introduce and in many cases encourage the audience to care about the characters in a story. If the audience knows about the characters from previous stories this allows the story teller to tell a story more quickly. So reusing an established villain allows the story to be told more quickly.
We can see this principle working in television series and movies. In the days before most movies were franchises, or franchise wannabes, a new set of characters was developed in most movies. Movies were typically two hours in part because it took time for the audience to learn about and emotionally connect with the characters. TV crime shows were typically an hour long because even though the policemen or private eyes were familiar to regular viewers the criminals and potential victims (damsels in distress) were new and had to be developed for the audience to care. In a half hour comedy all the important characters are frequently well known to the audience. This is a major reason that movies were so consistently about two hours, dramas an hour, and comedies a half an hour.
An exception is half hour hour anthologies, most famously The Twilight Zone, that tell a whole story about totally new characters in half an hour. This is an an exception that proves the rule, or at least illustrates why the rule works. In those shows something horrible normally happens to the protagonist and the story works better if the audience does not care too much about the protagonist.
I develop these points much more completely in another web page on why we have television series. This page applies the principles developed there to superhero comic books.
So recycling your villains allows you tell stories more quickly, why is that so important for comics?
Comic books are a very expensive medium, particularly by the standards of popular media. A twenty page pamphlet costs three or four dollars. If you read them at the rate of three per hour that is nine to twelve dollars per hour if they are read once. This means that there is a premium on telling stories quickly. Superhero comics do this using several tricks, or conventions. One is the trick of recycling the villains.
Another trick is the real world setting. Superhero stories are heroic fantasy. Many stories of heroic fantasy are set in fantasy worlds, for example Tolkien and his many imitators. Other heroic fantasies are set in space, Star Wars. This means that the writer must explain, and make the audience care about not just his characters but also another world. This takes time so fantasy is told in novels, often long ones, and frequently series of novels, traditionally trilogies. In the movies the fantasy is likely to be told in a series of sequels.
Superhero stories typically set a fantasy in our current world. The writer does not have to explain the current world to the readers. Because the reader understands the setting the story can be told more quickly.
There is a price to be paid for setting the story in our world. The space fantasy may gain a slight bit of credibility because who knows what might happen in space, or the future. It could be argued that the faster than light travel that most space fantasies use is not at all realistic. But note I said a slight bit of credibility.
Tolkien's and his many imitators take a different route. They reject all pretense of describing a real world. This may appeal to intellectuals because it keeps science, and more generally our knowledge of the real world inviolate by placing the action in a fantasy world.
The superhero story comes off as less sophisticated than other heroic fantasy, but once again it has the advantage that stories can be told quickly.
So because comics are expensive the stories have to be kept short, and this is done setting superhero stories in our world and recycling the villains. So now you know why Batman does not kill his villains, and perhaps a few things considerably more important than that.
Here is a link to my contact information..
History of the 4 major commercial universes DC, Marvel, Star Trek, & Star Wars
Elements of Star Wars inspired by
Lucas uses the initials of the Sith to credit his inpiration for that Sith Lord
Other Star Wars Pages
The first Death Star's floor space as large as surface of the earth.
Are the Targaryens the Welsh Media & Pop Culture Index.
Media & Culture Index: superheroes, Star Trek, fantasy, and much more including some serious stuff.
Radio Reception Tips
AM radio reception Tips Probably the most extensive collection of tips on the Internet and one of the most popular.