As Mark noted:
Another "disconnect" for me was Cooke's nonsensical decision to make Korean War fighter pilot Hal Jordan...get this...a pacifist. That's right, Hal made a point of refusing to shoot down hostile enemy aircraft, thereby adding to New Frontier's strange moral paralysis when it comes to its historical view of Communist expansionism and the clear need to contain it.
This was a jarring note for me as well, for several reasons. First, since Hal (in the NF) was a fighter pilot, whose father had been killed in WWII, it seems unlikely that he would have this pacifist streak. And second, if ever there was a DC hero who positively reveled in violence (if not killing), it was Green Lantern, possibly because Gil Kane could draw a punch better than any other artist:
That panel of course shows the Golden Age Green Lantern throwing the haymaker, but trust me, it was a rare issue that didn't feature Hal or his GL identity using his fists.
It is possible that Hal's unwillingness to kill in Korea marks him as suitable for the lantern and the ring, although it raises doubts about his suitability as a fighter pilot.
Early on in the Golden Age, DC's heroes were somewhat cavalier about the fate of criminals. Batman's cold-blooded response to the death of the villain in Detective #27 was, "A fitting end for his kind!" And those early issues frequently showed him tossing villains off rooftops to their apparent death. As I discussed in my (long and getting longer) post on Batman and guns, this changed after Batman #1, in which our hero kills monsters who were once men, and the driver of a truck with a machine gun mounted on the Batplane. As I noted, after that, Batman never shot to kill.
The concept of the hero not killing quickly became established for DC superhero types after that. I am not sure where this originated; it is well-known for example, that Doc Savage, a precursor to both Batman and Superman, used "mercy bullets" in his revolver that rendered his foes unconscious rather than killing (although he was quite willing to perform a little "brain salad surgery" to turn criminals back into productive members of society).
At any rate, this was codified fairly early in the DC Golden Age, and it lasted through the Silver Age.
There were, of course, several stories whereby the hero or his alter-ego was accused of killing somebody, but it was always a mistake or part of some other plot, as in Detective #249's The Crime of Bruce Wayne
But in Adventure #342, we got the real deal:
Star Boy, while on a trip to another world, sees a man kill an explorer he had befriended. The man then turns his gun on Star Boy, explaining that his goal was actually to kill the Legion member. With apparently only one chance to save his life, Star Boy violates the code:
He may be in the clear as far as the law is concerned, but not with his Legion buddies:
That is a pretty sophisticated argument made by Superboy. However, Star Boy's fate is sealed when Brainiac 5 demonstrates a way Star Boy (whose power is to make things super-heavy) could have survived without killing:
Star Boy was expelled. He and Dream Girl, who was not yet a Legionairre, joined the Legion of Substitute Heroes for the next year or so.
In Action #358-359, Superman was accused of homicide for accidentally killing a man whom he boxed against in an exhibition bout. He didn't know that a criminal mastermind had secretly set up the situation. But we did learn a little more about some of the "exceptions" in Superman's code against killing from Lois Lane's startling testimony:
So it's okay, as long as they're "dangerous aliens from space wearing human masks--condemned murderers?"
I will update this post as I find more stories that deal with the general DC code against killing.
Update: Green Lantern's code against killing is discussed by Sinestro, from Green Lantern #7:
Update II: The Avenger Code:
Update III: The X-Men Pledge:
Update IV: More about Superman. In Superman #171, our hero was confronted by two aliens with a very tough choice:
This story features terrific characterization. Superman, faced with the option of going against his code or allowing Earth to die, decides to kill himself by purposely exposing himself to Green Kryptonite. However, the aliens are not going to let him off that lightly, so they change the Green K to another element. They want him to kill someone else. Guess who volunteers?
Wow, terrific characterization for Jimmy. Later in the story, both Lana and Lois attempt suicide (yes, this issue went out with code approval), and in Lana's case she almost succeeds. In the end, Superman decides to "kill" Clark Kent by tying him up near a nuclear test blast. Of course, Clark really survives but he hopes that this will fool the aliens.
No such luck, but it doesn't really matter as the aliens weren't really planning on destroying Earth:
Yep, these are the same gamblers Rokk and Sorban, who were featured in the nutty World's Finest issue I talked about last year. They were also supposedly behind the Flash/Superman race in Flash #175, although we found out at the end two of Flash's major villains (Abra Kadabra and Professor Zoom) were impersonating the two aliens.