review of Captain America: The First Avenger, Steve Rogers is "a hero for whom decency, humility and self-sacrifice come naturally..." This is the guy that deserves a radioactive spider bite, or a crash-landed alien with a power ring.
He "has all the responsibility in the world; it’s the power that he lacks," Steven notes. He just wants to help people, but he doesn't stumble fatefully into that alien or spider or magic gem or power glove. He volunteers for an experimental procedure, and here is where we have a problem.
God only knows what was coming for Steve Rogers. (Literally. (1 Cor 2:9)) He could have gone on to live the life of a normal human, but normal humans change the world constantly. We change it in small ways; some of us change it in very big ways. We save lives, raise children, cure diseases, build cities... Small hands often move the wheels of the world, to use Tolkien's words, but Steve's hands don't remain small for very long.
As Rebecca Taylor put it (Why I am not a fan of Captain America), "...Steve Rodgers was experimented on to make a better soldier to help win a war. He was healthy and underwent potentially fatal procedures to make him Captain America." He undergoes a dangerous treatment not to cure a disease or repair damage but to make him more-than-human. Science gives him what he wants rather than heals or restores what he is. As Rebecca explains in another article (What is the Catholic view on genetic engineering?), the Catholic Church "distinguishes between therapeutic manipulation and
manipulation that simply alters the human genome for purposes other than curing genetic disease."
The ends don't justify the means. The means to this superhero were morally unacceptable and cannot be justified by whatever good he may do. There is a problem, though, with the Steve Rogers case: Steve Rogers.
Imagine that we make this case convincingly to Steve. He's a good guy, so he sits and hears us out. He agrees and regrets the procedure. His next question is likely going to be "Now what?"
Even if a reversal procedure was available, Steve wouldn't be required to try it if there was undue risk. Similarly, a Catholic that's been sterilized and subsequently repents need not undergo a reversal if the procedure represents an extraordinary burden or risk. (Though for many people today, it would not be an extraordinary burden and for very few, if any, would it be dangerous.) If the procedure is risky or not possible, then Steven remains the superpowered Captain America.
If he uses powers that he gained immorally, he is benefiting from that act - and giving scandal to others. (Who else might go through the procedure, even knowing it's wrong? I'll just confess it later and then, oops, I'm still stuck with superpowers.) On the other hand, if he does not use his powers in certain situations, he would allow people to die that he could have otherwise saved. Hopefully, Tony Stark can pass a communicator off to a good moral theologian.