The thing that gets me is that he had that AR-15 with (at least) two fully loaded clips of 15 rounds each in his truck. I can see him carrying his service pistol around with him, but an assault rifle? Either that was normal with him(in which case it says something about his mental state) or he packed that weapon with him that night for a reason. And that smacks of premeditation, not a crime of passion by someone who just ‘snapped.’
You have to wonder if there wasn’t something evil in that SWAT training that he had just received....
I don't think it would be all that unusual for a deputy in a small rural department. Could be a long ways from wherever one might be, to where he'd needed. But if that's department policy, then really should screen the folks before they issue that long rifle. Not that it would matter, he could have walked in to a gun shop and bought one just like it. Something which I'm thinking maybe I should do, sooner rather than later.