Here's your can of worms for the day: does Batman kill?
With a character like Batman - one who has existed for coming up on 80 years and who has been rebooted, retconned and reimagined more times than most people realize - it's a tricky question. You can go into the wide, deep sea of Batman stories and find a tale that will back up almost any assertion you can make about the Caped Crusader. I am not sure there has been another comic book character who has been represented so wildly differently by different creators - Batman is a grim vigilante, he's a loving father figure, he's a dedicated loner, he surrounds himself with the Batfamily. Grant Morrison actually did some great stuff with the various iterations of Batman's personality, trying to find a way to turn the massive left turns the character has experienced into an actual continuum of a human life.
The reason why I bring this up is because in a recent interview with Empire Magazine Batman v Superman: Dawn of Justice producer Charles Roven says this about Batman:
“Batman is not giving people a chance... He is more than a vigilante. He has become not only the cop, if you will, he has also become the jury and executioner.”
It's possible that Roven is being imprecise in his language. Maybe he doesn't mean literally an executioner. His follow-up quote doesn't make it seem like he's being metaphorical, though:
“He’s older,” stresses the producer, “[and] he’s seen the worst of what man can do. He’s been darkened by it, he’s tougher, he’s angrier, I guess.”
What Roven is saying makes it sound like Batman is out there actually killing bad guys. We haven't seen that in the marketing materials for the film (and I definitely do not recall this being made specific in the script), but that doesn't mean it isn't in the movie. So that leaves us with the question of whether or not Batman kills.
This is an easier question when it comes to Superman. No, Superman does not kill, and one of the main problems with Man of Steel is that it did not earn the moment when Superman broke his own code. Superman killed in the comics, but it was a huge deal. In the post-Crisis continuity he exiled himself to space after killing Zod and his Kryptonian buddies. Batman, on the other hand, has been committing murder since 1939. In his very first appearance he punches a bad guy into a vat of acid. He also throws a thug off a roof.
Again, this is where the differing versions of Batman confuses things. Batman as we know him today slowly took shape over the course of a bunch of issues of Detective Comics. He didn't even get an origin until Detective 33, six issues after his first appearance. His no guns, no killing rules would grow out of that origin, as well as a general softening of the character as he gained popularity.
Yes, Batman has killed, but should he? This is where it gets interesting, from both a fictional and a storytelling standpoint. If Batman kills his enemies he's simply not a good guy anymore; characters like The Punisher are 'heroic,' but they're anti-heroes, and we recognize that what they're doing - even if it's for the right reasons - are wrong. In the same way that we prefer our cops to not kill citizens, we prefer our superheroes to not commit murder. And make no mistake - it's murder if you're using deadly force without official authorization (Batman tends to operate simply in the 'assault' end of the spectrum). There are good stories to be told about whether or not Batman should use deadly force in a particularly sticky situation, but it seems to me that any iteration of Batman that proceeds from the concept that he's judge, jury and executioner is an inherently flawed iteration. He's simply not a good guy if he kills people, unless it's in absolute last-ditch self defense. And we don't talk about self defense in terms of 'execution.'
But there's also a larger storytelling reason why Batman shouldn't kill - because he would have no rogues gallery. If Batman killed, why would The Joker be alive in Suicide Squad? If there's anybody Batman should have killed by now, it's The Joker. An executioner Batman who lets The Joker run around is not only no longer a hero, he's also a shitty vigilante. One of the great dynamics of the relationship between The Joker and Batman is that every time Batman sticks to his morals he risks The Joker escaping again to take more lives. It's that sort of dedication to ideals that makes a hero a hero.
At least in the world of fictional storyetlling. I don't particularly mind that Osama bin Laden was killed - nobody needed that fucking guy sitting in a jail cell for the rest of his life as a symbol for others - but while we want some realism in our superhero stories we also want them to be representing our ideals. Yes, it's interesting to see Batman pushed to the limits, and the old, grizzled Batman from The Dark Knight Returns, upon which Zack Snyder is leaning heavily in BvS, is that Batman. But what makes a Batman driven to the edge compelling isn't that he falls, but that he hangs on.
I'll be very disappointed if Batman in BvS is a murderer. I can accept a dead body in Batman's past more than I can Superman's, but those deaths need to be few, far between and weigh heavily on Batman's conscience. Hell, if the basic dichotomy of BvS is to work Batman and Superman need to be on at least semi-equal moral footing, otherwise Batman is clearly the bad guy in the equation. And yes, a bad guy can be doing the wrong things for the right reasons - they're still bad guys. See Watchmen for an examination of that. And I know Zack Snyder is familiar with that story.