Why Zack Snyder's Wrong About Batman
4. How Batman's 'One Rule' Informs The Character
When Batman first appeared in the pages of Detective Comics #27, he killed. He also used a gun, and continued to do so for about a dozen or so more issues. It was only later when the character would adopt his famous 'no killing' rule, but it was that evolution in the character's history that made him such a unique force both on and off the page.
Any hero can kill. Most featured in Hollywood tend to use lethal force very liberally, and have done so for decades. When it came to comics, the pulp heroes of the Bronze Age, such as Dick Tracy and The Shadow, both used firearms and killed when the situation arose. Had the Dark Knight maintained his commitment to killing his foes, he'd just be another pulp hero who carried a gun; the fact he actually valued life was the thing that made him such a captivating figure.
Since his inception, Bill Finger and Bob Kane's creation has been riddled with flaws. His ideology is farcical in places, but that commitment to preserving life is what makes Batman so special. That in itself might even be silly to say, given the character's propensity to dish out cold, unforgiving street-justice on a regular basis, but it's true - Bruce Wayne takes to the streets so that no person may undergo the tragedy he befell when he was eight.
He doesn't view Gotham as a playground to tear-up in a militaristic motorcar, but instead as a city that can be saved. He funnels millions into anti-poverty programs, pretty much starts a family of his own, and abhors both killing and guns. One depiction that really understood Bruce's commitment to that last aspect was Batman: The Animated Series, which showed the moment Wayne retired from crimefighting in Batman Beyond when he entertained the thought of using a gun in self-defence.
In essence, Bruce Wayne's 'one rule' is a key pillar of the character. There have been some exceptions to that rule in the past, when all other options have been exhausted, but it's not something that can be dismissed with a brief shot of Robin's tattered costume.