There are a few possible answers:
* The most famous Batman villain who is a murderer is the Joker. He is known for his chaotic and unpredictable nature, and he often commits murder as part of his schemes. However, it's important to note that the Joker is not always the main villain in a Batman story, and he doesn't always murder.
* Other famous Batman villains who are murderers include:
* The Penguin
* The Riddler
* Two-Face
* Catwoman (occasionally)
* Ra's al Ghul
* Bane
* Scarecrow
To give you a more specific answer, please tell me:
* Which Batman story are you interested in? (e.g., Batman: The Killing Joke, Batman: Arkham City, Batman Begins, etc.)
* What kind of murder are you asking about? (e.g., who killed a specific character, who is the most prolific killer in Batman's universe, etc.)
Once I know what you're looking for, I can give you a more helpful answer!