Excuse me - This was not a "shoot/no-shoot" situation.That is simply a phrase to make it look like they have the most dangerous and demanding job in the world, and they do not ! It was clearly a "no-shoot" situation all the way, no gun should have been pointed, no finger on the trigger. There is no "decision" to make- it is common sense- not to mention the common law of the land.
The same law applies to everybody and this FBI Agent should be tried for murder. While I believe in the second amendment, the right of a citizen to bear arms, this guy needs a felony on his record so that he can never be hired as a policeman again, anywhere ! But it is possible the FBI will "protect one of their own" and keep him on the payroll like Lon Horiuchi ! After all the guy he shot they would call a "subject" .