Which one of these 3 do you see having the LEAST chances of becoming a villain, no matter the situation. Let's not look at the minority of showings for these characters since each have probably became horror like. Let's base this off of the majority.
Rank them from least chances of becoming a villain to 3 being the person who would become a villain first.
Least: Captain America. Yes, I know how recent adaptations have made Steve many things, but his origin story (if that hasn’t been changed too) was of a good hearted but weak young man striving to make a difference. He decided to serve in the only way he could, but literally offering himself. While recent years have changed many things, his original characterization was as far from evil/villainy as one could get.
Spider-Man: Peter is similar to Steve, just a bit more impulsive. Also a good cat that would require a significant push to make evil. He has faced real reasons and tragedies that would make lesser men turn, but he has still remained largely true.
Superman: This is the one character of the three that has the greatest potential for evil. For one, he is not even human ...he merely looks like one, but is anything but. There is something frightfully deceptive in that. He has powers beyond those of most heroes, and it is easy to see how he can decide to make thins ‘better.’ He could go to space, float about listening to conversations with his super hearing (hey ...if a man can fly a man can ‘hear’ in the vacuum of space), seeing with his super sight, and lobotomize ‘undesirables’ with his heat vision. He has great power, but he was born with it. It was not a sacrifice he made like Steve (sacrifice because the experiment could have gone wrong), and it wasn’t thrust on him through an accident like Peter. He was literally born an ‘ubermensch.’ It is only by fortune and chance that he was found by the Kents. Had the rocket landed in Nazi Germany (or as in Red Son, the Soviet Union, he would have been something totally different).
Lex Luthor is right. Superman is an absolute monster, and just because he has not turned does not mean he will not. It is almost a given. After all, he is a man-child with the powers of a small god....and like a kid with a termitarium, it is only a matter of time before he takes his magnifying glass and decides to crispy-crisp some insects.
Superman, by far, is the most evil thing of the three. He just has not turned ...yet.
I've always thought Spider-Man was a sissy. Cap is a slave to his morals, but you see him get pissed at times. Superman is a boyscout, but you'll see him cut loose on people that can take it.
Manchester Black tried that very same tactic. He cuts loose, against very specific opponents (Maxwell had to use Doomsday). But most of the time, using the majority of their showings, only one of these three are called the boyscout.
Spiderman would become a villain before CA and both before Superman. Superman from the mainstream universe was tempted by abstract forces and never failed. His story, the story of good versus evil is even the most powerful force in the DCU.
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For the most part, if this is arranged least to most likely to turn villain, as OP asks, I agree.
Spider-Man is used to standing up to people a LOT more powerful than himself and without the aid of allies, and putting his life on the line. Unlike Cap, he won't go along with anything just because some "legitimate" authority tells him to, either. He can only be controlled through his family, and, in most cases, it's been seen to that Pete's villains are kept in the dark about who Peter himself really is, let alone his relations.
Cap is too much on the side of secular law to be much more trustworthy than his superiors. What is legal is not always right and good.
As for Superman:
Justice Lords, Injustice League, Sacrifice, Public Enemies, Earth-2, Kingdom Come, and practically every story that features Lois Lane murdered, depicts a Superman who went mad and villainous from the hopeless sense of loss and rage.
I mean, it's valid to say that any could turn given a particular set of circumstances, or that none of them would because we've seen them hold the moral line on numerous occasions.
Pete had a famous arc specifically about this: Maximum Carnage. I think that's the closest thing to definitive proof that he'd never turn that I've seen for any of these three, but I'm also not as canonically thorough with my knowledge of the other two. And - it should be noted - Cap shows up in Maximum Carnage in what's probably Pete's darkest moment to provide moral support in his decision(s), so it's not like plenty of evidence doesn't exist for him as well (and Superman, as well, no doubt).
re: Superman, there's a difference between wanting to save the world from itself, and what happened in Injustice. He's as unlikely as any hero to actually turn bad. Injustice would be completely out of character though.
Killing Joke is an interesting look at this, because much like Maximum Carnage, it asks this exact question. I realize Batman isn't in the OP, but it's a nice analogous meditation on the question. Does it really only take one bad day?
I think Killing Joke has to be non-canon, because it's implied that Batman kills Joker at the end. But Gordon didn't turn, and he's arguably the focus of the Joker's experiment.
In the mainstream, Eclipso couldn't even touch him with his rage amping, Superman had to allow him to take over. Manchester Black forced him through an Injustice scenerio, and he refused to kill Black.
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