Jack hasn't been in Rapture so long. He hasn't gotten to the point of shooting first.

The guy is almost on top of him by the time he starts to wonder if that is a mistake.

He stands up quickly -- he's been spotted, anyway -- and his gun hand is the steadiest part of his body, pointing its barrel straight at the man's heart. (Or -- where Jack thinks a heart probably is, anyway. He's just a farm kid, he's making a wild guess. Only it happens that the guess is now -- and always is -- dead right.)

Jack's attempt to not look terrified is an utter critical failure. But--

"Your face is normal?"

What? It's unusual!