Unless there's more to this story they both sound like a couple of hot heads.
A hot head with a gun usually isn't going to end well.
If he needed the gun when he left work at night or had to some out from behind the glass he should have kept the it somewhere that would have prevented him from using it in a moment of anger. There have been news reports that he kept it in his belt.

If stopped using gas stations in the city, but years ago when I did I witnessed all kinds of rude behavior, cursing, and even tossing items around. I could see a person working behind the counter losing his cool, but you need to put yourself in a situation where you limit the trouble you could get into.