Originally Posted by
kalorama
Tony is controlling, and it's exactly that trait that has always made him leery of putting control in anyone else's hands other than his. After his "awakening" in that cave, he doesn't trust traditional power structures to do what's right with the power of his creations, be it the military, Congress, or SHIELD. We've seen that played out, again, ion every previous movie. He's steadfastly resisted giving control over his creations to someone else. His response to menaces of his own making is for him to make something to deal with those menaces precisely because he made them. Age of Ultron underscores this, as he was still gung ho to unleash the Vision on the world even after he'd made the colossal blunder of unleashing Ultron on the world. He's just not a "sit back and let this be someone else's problem" kind of guy, and Civil War never really gives us any credible, character-based reason to explain his sudden shift in worldview, personality. Like he tells Banner in AoU "We're mad scientists, buddy. We gotta own that."
If the Accords had been his idea and he were running it, then I could see him being all in, but Thaddeus Ross is exactly the kind of guy he'd never want pulling his strings.