How would he live with himself afterwards

These baddies can kill BECAUSE they are sociopaths, not the other way around. For a character like Batman or Spider-Man to cross that moral or psychological line would require a mental effort of staggering proportions, and would have an irrevocable effect on their psyche. Look at what happens to soldiers, post-theatre, or cops that have to fire their weapons. People with the law and morality and necessity behind them. The cost can be high.

Ironically, to resist the urge to kill might actually be the most realistic thing that a superhero can do…

Here’s the thing: a fireman won’t rebuild your house. Leaving aside any other moral or legal considerations, a superhero – a civilian/vigilante/community support agent – wouldn’t be any more or less a cog in the system than a paralegal, a clerk of the court or a prison guard. Part of the unspoken social contract that holds our societies together is the notion (or the hope) that everyone has a part to play, right?

For a superhero to cross that line – to kill in anything other than immediate (inadvertent?) defence of self or others – to kill in order to protect, preemptively, hypothetical future victims – undercuts that social contract. Emasculates the system that they might otherwise support. And that way, Authority lies. Or Doom, give or take ninety years.

Of course, there are stories or characters for whom such actions are a dramatic necessity – V (appropriately enough, given the day) – but for characters like your Batmans, your Spideys, etc., it’s a line to be forever avoided. Not because it sends the wrong message, but because it would have an ceffect on the character that, Cosmic Reset Buttons aside, they couldn’t take back.

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Not only that, but by hurting people I mean killing them.

So you’re one of the few guys around who understand this maniac well enough to bring him down before he does too much damage. Too much, but not no damage. Someone always dies, or very nearly so, every time this guys breaks out. Even you can’t be eternally vigilant because this guy has given you the slip occasionally as well.

However, let me congratulate you on sticking to your guns and not permanently taking this guy out of the picture. I know you’re so anguished and tormented by this turd killing so many people. I’m sure they feel a lot better that you haven’t become “just like him” and permanently taken him out of the picture.

Of course, what’s really at stake here is that this aforementioned villain is worth $$$, and you simply can’t kill him. Also, the people he kills largely don’t matter. The readers aren’t made to care for them. Their deaths just serve as shorthand for “this guy is really bad”, and once they’ve died, they cease to exist for all reasonable storytelling applications. So the villain gets to continue to exist, the writers get to repeat the same stories a few times each decade, and a breezy moral relativism is allowed to endure so long as the readership as a whole doesn’t think too hard about it.

Also, in one of the early New Avengers issues, someone at SHIELD says something to Spider-Man about superheroes being responsible for bad guys killing people after throwing them in jail time and time again only for them to break out. My reaction to THAT argument was that the system itself is obviously broken if the best it can think to do with people who continually break out of prison is to keep locking them in prison. It is a responsibility beyond just the superheroes… but NONE of them are morally superior because they refuse to kill. That’s a childish and inane concept that shouldn’t be put forth by anyone over the age of seven.

Superheroes pretty much would stop him, though.

The way I see it, it’s not that superheroes cling superiority by not killing, but if their policy is to capture criminals but not kill them, that’s a perfectly reasonable policy. They want to turn them over to the law because they’re acting to support the law.

Like with Batman people will get angry that he hasn’t killed the Joker, but there’s no reason Batman has to kill the Joker. If it’s so bad to keep him alive a policeman could do it, for instance. Just because a superhero decides to take on some responsibility for others doesn’t mean he’s wrong for not taking on even more responsibility. He doesn’t have to do any of it.