The novel Gladiator by Philip Wylie may or may not be an inspiration for Superman. It was published several years prior to Superman's first appearance in Action Comics #1, and sources I cannot cite from memory indicate that Jerry Siegel read the book before coming up with the character. The characters share many of the same traits, though not the same genesis, and the stories explore many of the same themes. If I were to make an argument that they are in fact the "same" story, I would have to found that argument on the despair that Hugo Danner (the Gladiator in question) must fight off throughot his life.
Yet Superman does not despair. Does he? The only reference to this that I can point to comes not in a Superman comic or movie, indeed, it's to a comic book in which Superman does not even appear. It's Endless Nights, by Neil Gaiman, and in partular to the story "The Heart of a Star" drawn by Miguelanxo Prado. In it, in the early days of the universe, the stars and nebulae and the things that come before gods get together to have a meeting. In one conversation, Despair tells the star Rao (around which Krypton orbits):
Wouldn't bringing life onto a planet that is inherently unstable add to the beauty of the life? If at any moment it would explode...Truly it would only be perfectly beautiful, a perfect piece of art, if one single life-form escaped. To remember, to mourn, to despair. (Endless Nights, "The Heart of a Star," pg. 76)
This would have us believe that Superman's defining characteristic is not flight or morality, but mourning and despair. He is alone among people who are like him.
Interestingly, I interviewed a woman named Jodi Chromey, whose affinity for Superman echoes this interpretation. She has two half-siblings, whom she is like but unlike in origin and appearance. So she feels an affinity for Superman and even got tattooed with the S-Shield because of this element of his story.
Danner in Gladiator spends his whole life trying to reconcile himself to his powers--gained because his father performs what would no doubt be considered genetic enhancement if the novel were written later in the cnetury. They cause him trouble and pain at every stage of his life, and the novel is a series of explorations of how Danner tries to cope with each new stage and the troubles it brings. It's a great book.
Hugo realized at last that there was no place in his world for him. Tides and tempest, volc anoes and lightning, all other majestic vehemences of the unverse had a purpose, but he had none. Either because he was all those forces unnaturally locked in the body of a man, or because he was a giant compelled to stoop and pander to live among his feeble fellows, his anachronism was complete. (311)
There's no place in the world for him. He spends a lot of time searching, and we have to wonder why he never finds one. Perhaps it is because he is given no clear agenda by his father, whose experiments created Danner's condition (which, by the way, is not hereditary, which actually separates it from genetic). Perhaps it is because he generally keeps his condition a secret, save for a few times. And when the revelation of his nature does not instill disgust or fear in his confidant, Danner retreats from the potential friend. He keeps expecting greatness, so much so that he does not allow himself to attain it. I'm reminded of a line from one of my favorite stories, Mefisto in Onyx by Harlan Ellison. The protagonist, Rudy Pairis, is also gifted in that he can read minds. Instead of a boon, the horror and ugliness that Pairis sees in others essentially keeps him unemployed and mostly friendless. Ellison says that Pairis "couldn't get out of his own way." I think that applies to Danner as well.
But it doesn't apply to Superman. I think first of the Richard Donner movies, in which Superman is given clear purpose by Jor-El, who sends him to Earth as protector and "light to showthe way." In contrast, Siegel didn't, as far as I know, give much reason for Superman's choice to fight crime and social injustice. Later, Mark Waid's Birthright would spend a lot of time exploring Superman's choice and giving sound reason for it. He does not dwell on despair, but on the fact that Superman comes to the decision himself, after seeing the injustice of the world (particularly in Africa).
For both Danner and Superman, the decision as to what they must do with their lives comes at the onset of manhood. This is when despair is most possible, especially without direction. Danner tries being a soldier, a lover, a laborer, a political "lobbyist", a banker, and other things. Always, his strength proves an obstacle that he cannot overcome. Superman, on ther other hand, splits his life into two parts: super hero and journalist. He chooses not one, but two paths. This may be the real need for the secret identity, which many writers have decried as unrealistic and unnecessary save for the tension it provides the relationship with Lois Lane.
What I have learned reading Danner's story is that the difference that comes with such great strength brings with it not merely responsiblity (as Stan Lee would have it) but also adaptation. Such power is not the provenance of mortals, but of gods, which Wylie's narration repeatedly invoke. The rest of us simply cannot handle it in one of our own. We require not equality, but at least the potential for a level playing field. We resent it if someone rises too high, and we must impugn their perfection. Thus the many jokes that subvert Superman's moral rectitude.
We may just look at Superman and Danner as two explorations of the cost of power, one optimistic and one pessimistic. But that's not all that's going on here. Superman is the story of an optimist, and certainly Danner can't get out of his own way, but their stories (their story?) are actually about adaptation to one's lot in life. Superman devises a way to live by disguising himself so that he may present the world his true face in two aspects. Danner tries to hide his nature, but gives himself no outlet. He reveals his powers only grudgingly, and then rejects anyone who offers solace. Superman does just the opposite. Again, Mark Waid's take on the character rings true: his Superman insists that he cannot wear a mask--if he shows his face, peopel will trust him. Danner hides, and his dishonesty is rejected.
A last thought, on gladiators: they're slaves. More to come.