To kill or not to kill, that is the question. Is it nobler to take, or spare the life of a monster?
For me, it depends both on the circumstance, and the players involved. I believe it is justified to kill in situations such as self-defense (or defense of others), as combatant to combatant in wartime, and legal execution of those who are simply too dangerous to ever permit in free society. To kill in defense, you or someone you are with must be under assault. To kill in war the victims must be legitimate targets that advance the war's objective, (civilian deaths must be avoided as much as possible), and executions must be staged in a humane manner by authorized personnel. Any killing outside these rules damages society and, I believe, the soul.
Superman's killing of Zod was justified because there was simply no other way to stop him. He was too strong to be contained, and there was simply no reasoning with him. I realize that this goes against Superman's typical "ultimate good-guy" image, but in this case, it was the "good guy" thing to do. Refusing to kill Zod no matter what would have been selfish. "I don't care how many people suffer or die as long as I don't personally get blood on my hands." They can still have the Man of Steel show forbearance in future episodes, when he may be tempted to take a life because it would so easy (and who would bring him to justice?) but he doesn't because this new scenario would be different.
Zod was a near-invincible conqueror who threatened the entire world, which I think satisfies the wartime requirement, what about more human criminals with a more scaled-down agenda? That would bring us to either defense from imminent harm, or legal execution. For me, any other type of killing wouldn't be right, and shouldn't be sanctioned. In the first season of "Arrow", Oliver Queen tended to drop bad guys right and left. His reasons why are understandable; these people had already escaped legitimate justice, and from what I recall, most everyone who was actually shot was an active threat to the Arrow or others at the time. Others are threatened, but not killed, as Ollie tries to coerce his targets into making amends first. However, I can understand the Starling City PD not liking the Arrow's activities. Even if most deaths are caused in defense of the Arrow's person, the fact remained that they would be threatening him if he hadn't shown to gather evidence or threaten their boss. He basically went looking for trouble, and appointed himself judge, jury and executioner to those who got in his way. At least the show acknowledges the bad precedent this sets when copycats appear which show less restraint than the Arrow. Another good point is where he stops killing by the second season. However, this only shows he had the capacity to do his job without homicide the whole time, but just didn't bother.
What particularly troubles me about the Arrow's vigilante killing was the mask and hood. When a regular civilian, or even a police officer kills someone, there is an accounting and investigation. The killer is expected to cooperate with authorities, who then verify their version of events. This can't happen with a masked vigilante. If a masked "hero" merely beats a crook up and leaves them for the cops, it's still not exactly proper, since they didn't stick around for a police report (which could hamper the felon's eventual conviction) but at least the felon's alive to reflect and possibly repent, plus the legal system put in place by elected officials gets the last word. No killing is the price the likes of Batman and Green Arrow pay for not having to answer to anyone.
In the season finale of "Arrow" I thought that when Slade's henchmen chose to take the miracuru drug, and go on the rampage, they chose the consequences, to be put down by whatever means necessary. Fortunately the Arrow and his allies got the cure to shoot them with, but absent that, anyone who was capable would have been justified in terminating the member's of Slade's army. Amanda Waller was certainly not justified in the drone strike (I wish they hadn't included that cliche); a few grenades, RPGs, and heavy machine guns could've done the job just as well, particularly when the henchmen were gathered together in the tunnels.
As for Slade himself, I think he would simply be too dangerous, even without the drug, to merely keep locked up. Even in real life criminals in prison don't always stay put, and even if they do, if they're killers they're a danger to anyone who works at the prison, as well as their fellow inmates. However, if I were Oliver, I wouldn't have wanted to put him down either, not before at least giving him the chance for sanity with the cure first. This is because the Arrow was simply too close to the situation. After everything Slade had done up to that point, it would've been too easy to kill him out of hate. Justice would've been better served by some Joe-shmoe executioner just doing his job. We see a similar scenario with Luke Skywalker in "Return of the Jedi". When he finally takes down Darth Vader, he refrains from delivering the killing blow, because he had become dangerously close to doing it just out of hate, rather than justice and saving the galaxy.
So that's my thoughts on justifiable homicide. Do you agree with my musings or disagree? I'd really like to see other people's thoughts on this one.