Re: If the author wanted a "killing is always wrong" shpiel, he picked the wrong villains.
Not necessarily above all else, but the "no kill" rule is one they don't break. I can't really fault them for it, since it's not like its their responsibility to kill the Joker if he needs killing so badly. It's not only with the no killing rule that they're following their own idiocyncratic code of conduct. They could kill the Joker if they wanted; they could not stop crimes on the street if they wanted.