It would be bad, and traumatic, for Peter or Harry to kill Norman, and even if he did it would result in major fallout and I'd never want to see him make a regular thing of it.
What I'm arguing here is that simply because the fight was over and Norman lost, sure the immediate danger was gone, but what other options are available when the law won't touch it and nobody seems to care about his bad reputation? Even if Batman never kills the Joker, he at least doesn't just walk away and let him rampage unchecked.