No one so far has mentioned the intrinsic morality assumed in most (but not all!) of the major superhero titles and narratives.
For much of their post-WWII history, mainstream superhero comic books have held to the belief that no human being (or sentient being) is completely beyond hope of redemption and rehabilitation. Most of the major superheroes have dedicated their lives not merely to stopping crime or villainy but more importantly to helping those in need and to helping bad people become good people. They are not looking for excuses to slay the dragon but instead driven to inspire good in everyone, even in criminals and dragons, and to act as role models to their best of their abilities. Not "smite the sinner!" but "Go forth and sin no more." And the metaphysics characteristic to most superhero worlds means that inspiring good in just one villain could have a palpable effect on the world.
All of us can think up the exceptions, of course, such as Marvel's The Punisher and sometimes Wolverine or DC's The Spectre and certain Vertigo titles.
However, this faith in redemption and rehabilitation is why The Batman continues to hand The Joker over to psychiatrists (as well as many other Bat-villains) rather than killing them; it's why Superman still tries to inspire nobility in Lex Luthor, and why most imaginary stories about a dead Lex Luthor includes Superman's mourning that he was never able to inspire Lex to live up to his potential greatness. Spider-Man responded to the first death of Green Goblin with regret for a death rather than satisfaction in the seeming death of the man who killed Gwen Stacey. Captain America felt a moment of pity for The Red Skull when he found out his tragic origin -- something which outraged The Red Skull, who had wanted to provoke outraged hatred not grim pity from the Captain. Green Lantern would hand Sinestro over to the Oans rather than kill him, and they in return would imprison Sinestro rather than execute him. Professor X always tried to rescue bigots and demogogues from mutant extremists rather than let them kill them blood for blood. For most of their comic book history, both Thor and Wonder Woman have tried to avoid killing any of their mortal foes (monsters are another matter), though both of them avoided second thoughts when the moment came when they felt they had no choice but to kill.
Admittedly, not every hero uses the same tactics to try to provoke rehabilitation and redemption. Superman and Captain America have both tried to inspire it with sheer nobility and compassion, whereas The Batman has followed more the "Scared Straight" path of frightening amateur criminals into choosing another path for their lives, and Spider-Man has tried reason and simple human decency. A number of times, the X-Men titles have criticized human nature by pointing out how often people have chosen fear and petty bigotry over Professor Xavier's reason and ideals.
It is not uncommon to have a quick one page scene within which a major hero runs into a petty criminal who had been inspired to reform and discovers that the person has remained reformed.
The one problem with all this is that, in serial storytelling that centers on crimefighting and good versus evil, rehabilitated and redeemed criminals usually disappear, so no one remembers them, while the ones who remain criminal and/or evil continue to reappear and gain reader recognition. Those few who try to become heroes suffer from the fact that there are only so many titles out there, so they either disappear or end up returning to their villainy -- or they disappear but a new villain takes their powers and villain name so that the company can maintain its copyright or trademark control over that name.
Superman does not mutilate, murder, or rob others of their civil rights -- including their 2nd Amendment rights to have superpowers -- for the same reason that most (not all!) superheroes avoid this: because he embodies and lives in a world in which the greatest drive for a hero is not to destroy evil but to turn evil to good.