Adrian kept a straight face and showed no emotion and no change in facial expression as the deputy director spoke. She sounded heartless but she was good at what she did. The soldier listened carefully to the details of what the serum was doing, in the simplified terms Director Fury gave it made better sense to him. He was visibly relieved when he found out he wasn't going to be executed.
When the Avengers were brought up and he was told he'd be joining them he said "Thank you, sir." Adrian had expected a firing squad or something along those lines. A second chance was the last thing he'd expected. He damn sure wasn't going to waste it or screw it up.