Salvation was within in his grasp.
Tony truly believed that. He watched the Gem glow as his desire to do more consumed him. The Witch showed him his fears. And this cradle before him was the answer. So Ultron was a mistake. No need to keep wagging that in his face.
Though the way Steve looked at him like someone just spat on his mother's grave – that cut him deep.
But in his heart he knew this was right. Before he could jumpstart Operation-Hopefully-Lawful-Good-Bot his arm was wrested away so forcefully he thought his humerus was about to snap.
"Are you nuts?"
Tony bit down a surge of irritation whenever Cap was red-taping his work. Stubborn and old-fashioned, he didn't expect Steve to understand this, what more approve it. The least he could ask for was some respect in his judgement and capability, and trust that he'd do the right thing.
"First you give us Ultron, and now this?"
Two minutes later, Thor brought Vision to life. Steve didn't bring the issue up after, but Tony knew the hatchet was still out in the open.