The biggest difference between Stark and other superheroes, both in the Marvel universe and elsewhere, is that his goodness is not instinctive. Superman is reflexively good, Spider-Man spends his life making up for one weak moment, and Bruce Wayne often seems to be incidentally wealthy as a byproduct of his efforts to improve life for everyone. Cap was born good, Thor was born good, Bruce Banner was born good. They’re certainly not perfect — crises of conscience arise over whether these guys want to get involved. “With great power comes great responsibility,” and so forth.
But Stark, as Robert Downey Jr. plays him, is a reflexively selfish, self-promoting, ego-driven person with a genuine tendency toward bluster and rudeness. What fascinates about him is that the power comes first and the decision to become good comes later. He was rich and powerful before he was decent, as the opening moments of this film make clear; he gives of himself by conscious choice, by teaching himself a kind of ethics that don’t come naturally. He does it reluctantly, always for a complex combination of selfish and unselfish reasons. Until you hit him close to home, he’d always rather stay out of trouble."
Armor And Anxiety: Tony Stark Is The New Captain America