Hollywood likes to perpetuate this ideal that heroes can do whatever it takes to save the world -- short of killing someone. If they make that step over into murder, the heroes are supposedly no different than the villains. It's a nice thought, but it becomes problematic in practice. Not killing the big-bad is often one of the most selfish and irresponsible things a hero can do. Case in point...
I don't know if you've watched the Tim Burton Batman movies lately, but they're nothing like you remember them. For one, Batman has no reservations when it comes to murdering people that rub him the wrong way. He's responsible for The Joker's death at the end of the 1989 Batman movie, and is tickled god-damned pink when he blows up a bad guy with dynamite in Batman Returns.
But at the end of that sequel, Batman arrives at the nick of time to Catwoman out of killing Max Schreck, played with typical wide-eyed mania by Christopher Walken. Keep in mind, Schreck is the same guy who attempted to kill Selina Kyle -- that's why she became Catwoman. Even so, Batman pleads with Selina to spare the life of this high-ranking bad guy, like Schreck is somehow less dangerous than the dim-witted goon he obliterated without a second thought. Worse still, Bruce Wayne unmasks himself in an attempt to appeal to Selina -- so now Schreck knows his secret identity and could easily ruin Batman forever until Batman Forever.
Then again, that might not be a problem because Schreck immediately produces a gun and attempts to kill both of them, proving himself to be the only competent person in the movie. Catwoman had to use most of her nine lives to electrocute Schreck -- if wasn't for her actions, Gotham would be without its Dark Knight. The city would then fall into chaos; Bane would hock Venom to kids on the street, and the Riddler would send everyone really annoying chain letters All because Batman couldn't learn a lesson from his young self -- he didn't have to kill Schreck, but he didn't have to save him.