Stark was contracted by the government to create weapons, and has spent his time as a superhero cleaning up messes he made. His support of registration is easily attributed to guilt, but he should be turning his effort into "arc reactors for all" type philanthropy if he believes in being accountable. But he doesn't, because he doesn't trust what the governments of the world would do with free energy, yet, he's fine giving the ability to deploy gods,because it takes the responsibility away from him if another Ultron happens. The whole thing stems from Tony's guilt, and the desire for SHIELD to control all the toys.

What Cap symbolizes, and Thor, Hulk, Vision, and Scarlet Witch are, is a disruptive idea of personal responsibility. They have the power to change things based on their personal ideas of what Good Is. The fear of what they can do stems from our own inability to do this, and this primitive need for punishment to scale if someone fails in trying. It's why dictators can kill millions and be left alone, but a guy charged with child abuse because he turned 18 and his girlfriend was 17 can never own a house in peace or find a decent job. We turn a blind eye to modern day slavery as long as gas and iPhones are cheap, because we know it's too big of a problem to solve. But the moment we get a whiff of a local judge taking bribes, we bring the vengeance of a nation down on them. If the Avengers have the power and resources to remake the world, what if they let Those Oeople we don't like still exist? It can't mean we're wrong, or not the center of the universe,of course it's the super powered people who are wrong and a danger to us all.

Then there's basic strategy. You ultimately want smart weapons to have as much agency as possible to be effective. And threat wise, it's a self governing market, for every Ultron there's a Vision. Casualties can't be avoided from walking down the street now, much less from natural disasters or institutional failures. The MCU is now a world where there is just an extra layer of natural disaster level events to contend with. Police forces should be within their right to employ super powers, but everyone with powers shouldn't be regulated by a police force.