Re: If the author wanted a "killing is always wrong" shpiel, he picked the wrong villains.
"We do not kill" is a perfectly acceptable line. Someone being dead makes it a little harder to say you're sorry when you get proved wrong after all. Not to mention that the heroes would be taking the place of the judge if they did, and in Batman's case at least, it's something he avoids doing. He does the police's job, but doesn't allow himself to break the separation of powers rule.