Last year, Marvel Editor Tom Brevoort revealed that there's one story that many writers have pitched to him that he has always rejected. In response to a question by Kiel Phegley of CBR asking about the worst idea that he had ever rejected as an editor, Brevoort explained that there was one line that many writers have been eager to cross.
There's the one that half the people in the industry have pitched at one point or another: the villain who's so bad that Spider-Man has no other choice but to kill him. Taboo is such an attractive thing, I guess, but there are certain lines certain of our characters don't or won't cross, so it's attractive for people to think about the situations in which we might push them past that point. But most often, when this story is pitched, the writers haven't gone any further -- they haven't considered the after-effects, what killing somebody does to Peter Parker, how it changes him, what it does to his Spider-Man career. They're only truly interested in the shock value of the moment – and that's worthless without proper and compelling follow-through. It'd be the easiest thing in the world to suddenly have Spider-Man start killing people, but it would be a violation of his character as it's been established over the years. And there might be an instance when Spider-Man was forced to make the choice to kill -- but that would be a profound, life-hanging moment for the character, one that you might not really ever be able to get him back from. So you don't want to go down that road casually.
In the comics, Spider-Man has sometimes accidentally killed people. But Brevoort's answer was in regards to something premeditated, rather than something like Charlemagne's fate in Spider-Man VS Wolverine, or even the death of Gwen Stacy. While it has been strongly implied that Gwen died because Spider-Man accidentally snapped her neck by webbing her while she was in freefall, I am unaware that any technical cause of death has been given for her. And if he hadn't webbed her, she still would have died, so he only shortened her life by a few seconds, if that.Also, in general, people who come to me looking for key super-villains for either their new villain or their lethal hero to kill, in order to "prove that he's a badass." These, too, tend to be throwaway killings, meaningless and not actually accomplishing what these writers think they're accomplishing. Ironically, these tend to be the same guys who come back months later, looking for other important villains for their characters to fight. If you kill off all of the villains, then the picking become slim.
As a SHIELD agent, Peter Parker's father almost certainly killed people. Flash Thompson killed a few insurgents in Afghanistan in Amazing Spider-Man #574, before he became Venom. In the real world, heroes do sometimes kill. And when situations are screwed up enough, sometimes they fail to save the day. It reflects poorly on Peter Parker when it's made explicitly clear that he doesn't have the strength to do what many people have been forced to do in both the Marvel Universe and the real world.
It's an unsatisfactory answer to a question that has to be asked: Should Spider-Man kill a villain like the Green Goblin or Carnage who will almost certainly take innocent lives if allowed to stay alive? Anyone who has read enough Batman comics has similar questions, especially when it comes to the Joker. Now there's a shamelessly meta reason Batman will not kill. DC wants the ability to tell new stories with the Joker. And that's the main reason Spider-Man doesn't kill. Marvel likes having recurring villains and it's difficult to do those in a series in which the hero finds a permanent solution to all the prison escapes by super-powered criminals.
A counterargument is that Spider-Man shouldn't make a habit of killing, but that it could be interesting to put him in the position for just one story. But where do you go next? Once he's crossed that line, it will be easier to do so again in the future. There is also the impression that if he isn't put in the same position in the future, the writers just aren't pushing him hard enough. And when a subsequent atrocity inevitably occurs in later issues of the Spider-Man comics without the wallcrawler crossing the same line, detractors will ask whether the previous situation was so much worse. It would draw a lot of attention to storytelling decisions made by the writers and editors, which makes it difficult for the reader to agree that this is a decision that this character might reasonably make.
Under the right circumstances, Peter Parker would probably kill a bad person. I don't think Brevoort or anyone at Marvel was disputing that, although there is a certain integrity to someone who will not compromise even if innocent people will die. That would be a different direction to take the series, dealing with Spider-Man's guilt when his decision to spare the life of a madman has led to the deaths of innocent people. During his runs on Peter Parker Spider-Man and Spectacular Spider-Man, Paul Jenkins often put Spider-Man in really difficult positions on the issue of life. So that might be one thing for the writers to consider, placing Spider-Man in a situation in which any other superhero would have a different response. Although that may be too controversial.
Brevoort's response touches upon a theme of this essay series. Meaningful decisions for the Spider-Man comics should not be made casually. If someone wants to do a story in which Spider-Man is pushed into taking the life of a villain, they had better be willing to deal with the long-term consequences of that. They should also have a vague idea regarding what their successors on the title would do with that development. It's irresponsible to just do it as a way to give a story a memorable ending.
The ramifications could be interesting, but you need a writer who wants to tell that particular story, and is willing to devote the right amount of time and effort into the long-term and short-term consequences. Every page of a comic book is valuable real estate, usually representing at least a day of work for the artist. This development would be one that shouldn't just be glossed over. But you also have to be careful to make sure that it doesn't overwhelm everything else that is going on in the title.