Some people don't know when to stop. What boundaries are. The prankster here found this guy's boundaries. The victim felt fear, and reacted in his way. Do I get to draw the line in the sand where violence is the right answer? No. Judges, Juries, and lawmakers do.
Do I feel personally that this gentleman defended himself correctly? It's a thin line, but yes. As I said in another comment the guy probably ended up in high crime areas on a regular basis and a gun might have been necessary for those situations. So that's the defense he had on him. It's not like we all carry a selection of weapons and deterrents that we can choose from depending on where we are at any given time. We carry what works for the worst situation we encounter.
As a delivery driver myself I sympathize because I have a feeling this wasn't this guys first bad interaction with another individual. If he continues driving, it most certainly won't be his last.