We are living in Batman's America. Americans gravitate towards mediums and tropes that grew up with us: blues, rock, hip-hop, cowboys, movies, baseball, and superheroes.
Superheroes deal primarily with the question "What is right and how do I do it?" Any society that has the notion of "sin" is constantly striving towards a better answer but usually with the benefit of hundreds or even thousands of years of cultural heritage. We are brand spanking new as a nation, we have no precedent, and we're terrified. It's why we're so desperately religious and it's why we love superheroes.
Let's back it up and talk about the Greco-Roman pantheon. Gods, goddesses, demi-gods, mortals, muses, demons, and every manner of creature existed in the collective consciousness, alongside agreed upon back-stories and characteristics. Poets, historians, and playwrights adopted and interpreted these characters, sometimes tweaking their back-story, sometimes adding to their personality, always striving to express via shared symbols some collective crisis. Not unlike superheroes.
Superman, the ultimate icon of our own pantheon, first appeared in comics in June of 1938, just before the inception of WWII and reached the height of his popularity in the 40s and 50s, after the end of that whole business. In Superman's America, the answer was clear: kicking all that Nazi butt was right. Duh. The economy was booming, everyone was having lots babies, we'd done good and Superman was our guy. And no one has liked him since. Seriously, no one wants to read about Superman and no one really likes his movies. Countless reboots have tried and failed to make the dude relatable but the fact remains that he's perfect and he's literally an alien and no one cares.
Since Superman's America we've gone from the Korean War to 'Nam to 9/11 and of late have been embroiled in a really expensive and baffling conflict in the Middle East that the average citizen could barely begin to explain let alone conclude conclusively that the whole thing was "right." We've plunged headfirst into a bummer of a recession and no one is all that happy.
Now we want Batman. Not Adam West Batman, not Tim Burton Batman, but gritty hoarse Chris Nolan Batman. Christian Bale's Bruce is obsessed with the question, "what is right and how do I do it?" and, godammit he is trying to find an answer (and spending lots of money in the process). But he's not always sure and he's probably insane. He often wants to stop and plenty of Gotham's citizens wish he would. Batman believes in a Gotham that is ultimately good but plagued by the unnatural disease that is evil. For better or for worse, Batman is obsessed with becoming the antibiotic.
To the franchise's great credit, all of its best villains are also concerned with our question. Joker lives to show the world that our morals and laws, which we desperately hope will point us towards Right, are fabrications. Any person can be broken into evil, and any instinct to do "right" is unnatural. Harvey Dent, Gotham's "White Knight," is disfigured during his dogged quest to purify Gotham. Two-Face switches Jekyll and Hyde style between good and evil, with the decision left up to the flip of a coin. Catwoman, or at least Anne Hathaway's Catwoman, sees a world in which people readily screw each other over to get ahead. Maybe she'd like to do good, but it's not economically feasible.
With all these swirling questions about right and wrong and no satisfactory conclusion forthcoming, one can almost see how the Dark Knight Rises would be a savage psychopath's choice as the stage for a shooting. It's Batman's America, we just live in it.