If the author wanted a "killing is always wrong" shpiel, he picked the wrong villains.
Zsasz and the Joker are the only two Batman villains for whom the "no killing villains" rule makes absolutely no sense. (Zsasz, for those of you who aren't familiar with him...the whole reason he's in Arkham is that he's a serial killer with a compulsion to carve a new scar onto himself each time he kills someone. Note that he's keeping count in Roman numerals, up to groups of five, after which he starts a new group somewhere else on his body.)