The older guy man slapped the trigger puller.
The younger trigger puller escalated by pulling his gun. Why? I do not try to understand the insane.
The older guy was retreating and pulled his mace as a last resort.
Issues: how to defend yourself against a lunatic out for blood or a trophy?
What if they are marching through your neighbourhood at 11PM?
The older guy also had a firearm which he did not attempt to use. Maybe because he's a decent guy, or maybe because he had the mace in his right hand which was the hand he would have to use to draw his firearm.
Ego was hurt or he figured that the slap justified execution if he was carrying out a plan, imo. Murder either way. Any sort of drug could also could ease you past that fine line of self control, too. And the guy is said to be an atheist so there is a another guard rail he didn't have. About all the reasons I can think of.