Re: If the author wanted a "killing is always wrong" shpiel, he picked the wrong villains.
No. They think killing is wrong. Thus they actively prevent other people from killing beside just following that rule. It's not a matter of not wanting to get their hands dirty. Which is why Batman standing aside and telling Jim Gordon he wouldn't stop him from killing Joker at the end of NML is grossly out of character, unless you assume Batman had guessed Jim wouldn't do it.