Assuming Pale's summary is wholly true, we can make an assumption that this Batman here is someone who's been strayed from his own forged path, for he might as well have a no-kill rule at first, but it's been nulled over time due to experiences and some PTSD/mental issues, yes?
Batman is the hero where killing might have come easily for him, so for him to exert control over himself and justify his mostly illegal vigilante activities, a rigid set of codes is needed. A Batman who strayed from his own expectation of himself is a challenging one to write.
As you said, it's an interesting idea to explore in this particular movie. His idealism and morals evolved into a Machiavellian pattern of thinking, based on his field of experience, and it's something that is very human - juxtaposed with the fact that this human is battling a God-like alien. But it is something I feel that is exclusive to this story only, and has a possibility to be detrimental to future story lines/directions.
Red Hood is just an example. My point being, I have no gripes with Batman doing killings - or branding select types of criminals - and I value any interpretations of the character that feels he needs to, but in this universe I personally think it is unnecessary. I just don't want Batman to be painted as a sort-of Punisher figure when there's many other characters who could function in that kind of role, to put across certain points as intended by the writer/director.
Especially, in the movie universe where the company has all the rights and able to explore and develop characters other than proven money makers.
