The noises Bucky makes in this scene kill me. The desperation in his grunts and fear in his yells of pain. He’s trying so hard, so damned hard, to complete his mission, even though there really is something inside of him that recognizes this man, but he just can’t forgo the mission. He must be so afraid. Not even for his own life really; fear of what’s going to happen to him if he fails this mission and lives. He has no idea that Pierce is no longer a threat to him so there is terror in all of his movements and struggling against Steve.
And the torment that must be going through Steve because he’s the one causing him to make all those awful noises and to fight with all he has against him because he’s so terrified of failing.