The Air Marshall did the right thing but it's unfortunate that he has to live with the knowledge that he killed a man.
Yes, that's true, even though he was certainly justified, it is difficult to take a life, and probably more difficult when you find out the threat really didn't exist, unfortunately.
I wouldn't worry about him too much. With all the time and money the government spent training him to kill, I'm sure they found time to include a class or two on coping with the act of gunning down civilians.
Hey, the feds at Ruby Ridge and Waco were just doing their jobs too.