There is an adage - don't ask me from where - that says 'Only the powerful an afford to be merciful.'
I think this Batman exemplifies this - Superman can afford to be hold his punches because underneath he knows (even when powerless) he has the greater 'power' (either ethical or physical) although he might argue the point. Batman doesn't have that 'luxury'. He knows that all that stands between him and the bad guys is himself and he knows that rats have nasty bites.
