I agree with the points you raise, but that's not the argument I got from kitty's reply. If there is such a thing as carrying on a moral duty above the law, it applies to the cops as much as it does to Batman or as much as it does to anyone else who might ever be in a position to carry it out; and if that 'moral duty' is to kill the Joker, then it falls to them as well. (But again, I find the idea that killing someone as a moral duty confusing at best, and entirely irrelevant to Batman and Joker.)