Waving a gun at somebody to intimidate him/her/it
is illegal everywhere, so far as I know. It is called "brandishing a weapon", or "brandishing a firearm". If he really felt his life was threatened by the trucker, he should have called 911, and tried to escape while he was online to the dispatcher. If he felt his life was in immediate danger, and shot the trucker, then he'd have a runaway truck and who knows what would happen then? Likely he was just pissed off and thought he could prevail against the trucker in what was really a minor dispute.