Okay, then let's look at it from another angle.
It's only recently that it became "chic" for heroes to kill. Or, to cede you your point, let's make that "kill again." For decades, decades during which most of America discovered superheroes and formed an opinion of who they were, they did not kill. (And it's worth noting that most people did NOT discover superheroes through comic books at all, but through other media...which is still the case)..
More accurately they did not kill things that had a 'soul'. In his first appearance Superboy tried to kill Bizarro with Green K and latter did kill him with the remains of the machine that created him curing a girl's blindness in the process. Then you have the ocxational self will machine criminal that superman happily turns into scrap metal because it is 'not alive' These cop outs are only worse than GI Joe cartoon where no one every freaking dies in combat.
I also think you need to take heroes in the context of their times. Nobody would blame Captain America for killing Nazis, any more than they'd blame their dads for doing it. It was a time of war and anyway Nazis hardly count as human...if I met one today I'd probably kill him myself. And in the old Westerns, shoot-outs were (1) sanitized affairs with lots of noise but almost no blood..."Aghh... ya got me...PLOP!" and (2) pretty clear-cut in terms of good guys and bad guys.
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Not always. Little Big Man case in point. Also the Spagetti wester was natorious for having 'heroes' who were just as bad as the villians. Watch the
Good, the Bad, and the Ugly some time for an example.
By contrast, Wolverine...at least when I read the books in the late 70s...killed without much provocation. (There's that infamous scene where he sneaks up on a guard and "snikt"!, which turns Nightcrawler's stomach). And in the case of the Punisher, it's made clear in many stories that he's a loose cannon, a self-appointed executioner who is as much of a menace as the men he hunts down. He's not a hero in any normal sense, though I guess he appeals to kids with anger issues.
Of course by the late 1970's the Anti-hero had become popular as again best demonstrated by the likes of Rambo, Dirty Harry, and the countless man with no name cowboys running around. The Calvery lone=g a good guy was now a villian helping to destory Native American tribes and covering up their screw ups by court marshaling those who knew the truth.
In a way it was a whole back lash against the Vietnam war and Watergate messes. A war where it was
Amercian soldiers seemingly shooting helpless prisoners and burning villages (Mei Li). A president who though he was above the law. Rumors of an FBI head named Hoover who had a file list that would have made Himmler of the SS green with envy. As early as the 1960's you had the decline and my 1970s it became mainstream