It really bugs me when I see fans talk about Civil War like Bucky is the be-all and end-all of Steve’s motivation. It’s like they saw the first trailer (which does put Bucky at the centre of the conflict) and constructed their interpretation of the entire movie on that alone.
Bucky is a huge part of Steve’s motivations, but he’s not everything. In fact, treating him as such is kind of a disservice to Steve and his team, in my opinion. Because Steve’s fighting for autonomy – yes, Bucky’s, but also Wanda’s and his team’s. He’s fighting against government regulations that are not guaranteed to always put vulnerable people’s needs first. And then, when the winter soldiers are revealed, he’s fighting to stop a threat bigger than anything the world has faced before. Because he knows how dangerous Bucky is, and how evil the people who did this to him are.
When Zemo’s plot is revealed, that’s when he fights for Bucky – to save Bucky from the consequences of his decision to lie to Tony about the fate of his parents. But it took most of the movie for Steve to get to this point. He didn’t refuse to sign the Accords because of Bucky. He didn’t lead the fight on the tarmac because of Bucky. He broke international laws to protect Bucky and extract him alive from Romania because those laws were unjust. If it had been Wanda or Sam or even Tony, I really believe Steve would do the same. Because it’s wrong to apprehend someone and execute them without a trial, no matter what the governing body that sent the Avengers in says. Which is why Steve didn’t sign the Accords in the first place.
Was he being selfish? Yeah. Arrogant? You bet your ass. It’s incredibly arrogant of him to think that his judgment is the only one worth trusting. (Sidenote: it’s also incredibly believable given what he’s seen. Hello, trauma response!)
TL;DR: while it’s true that by the end of the movie, Bucky comes to eclipse his other motivations, Bucky was not the number one motivating factor for Steve’s actions from the get-go.