approaching a vehicle makes most law enforcement people very nervous and they are on edge to start with. but to fire a weapon when no opposing weapon was in view, contradicts most agency guidelines and I think the agents will have alot of explaining to do. generally you can only apply lethal force when you are facing a life threatening force (or you believe the individual may cause life threatening situations to innocent by-standers). all in all, (with the current info), it sounds like a major screw-up and hopefully the FBI will take the high road and make best efforts to minimize the impacts to the man and his family.
edit = and yes, many officers now approach vehicles with their hands on their weapons and may even draw it, but there's a big difference between drawing it and firing it.