I reckon he’s just frightened. His whole world has just fallen apart around him. He knows Steve hasn’t been trying to kill him all along, but he also knows he might be about to die anyway.
And he’s just found out that he has a name and someone unwilling to kill him and he doesn’t know why. He’s almost failed a mission for the first time, and unless Steve sacrifices himself he knows Steve is going to win and he doesn’t know what that means for him afterwards. He doesn’t know that HYDRA is gone or that Pierce can’t punish him, and he isn’t sure if his side is the right side or why he recognises this man and couldn’t just shoot him in the head when he had the chance.