Bucky Barnes: Captain America’s first shield
The more I rewatch (because that’s never going to stop happening), the more I see what was enhanced in Bucky. When he’s liberated from Zola, the first questions he asks are entirely about Steve’s well-being. He, the man who was beaten and tortured and imprisoned, wants to make sure his friend is all right. He wants to take care of him, even when he can barely stand.
So here’s my expanded theory: the serum takes what makes a man good or bad and amplifies it to the Nth degree. So Steve, who is just and good and fair-minded, becomes even more so. But Bucky? What do we know about Bucky? We know that his first and foremost priority in New York was to keep Steve safe. His protective instincts are epic, and in the deleted scene, he’s doing the same for the rest of the 107th.
Bucky is a protector. It’s what he did back in the day. He protects that which he considers his, in this case, Steve.
So when this corrupted serum was put into him, it took this loyalty and protectiveness and turned it up to eleven. And not just the emotions, but upped the ruthlessness with which he protected that which was his. If someone tries to kill the person that Bucky is protecting, he shows no hesitation in killing them first. He arms himself to protect Steve.
And this carries over to the Winter Soldier.
Not the loyalty, because that’s why they need to use their machines. But the fact that he will kill anyone who is a threat to his handlers. He isn’t loyal to them, not by choice. They use their machines to change his focus and make him protect them. He does everything he did for Steve for them now, because he still has that impulse to defend. It’s just that he’s now defending the people who turned him into a weapon. They took his mind and made him theirs, but they couldn’t change the person he was before, the protector.
The definition of shield is “a person or thing providing protection”. Long before Steve Rogers was Captain America, he already had a shield.
This just makes the ending sadder when he realizes he is attacking the very person he once was so determined to protect. Yet he overides that conditioning.