Part of the answer is adrenalin. (More on that later.) But, essentially, when you work, day after day, in an environment described as "eight hours of boredom, punctuated by moments of stark terror," your body's physical response starts to cause you to react to EVERY perceived threat to your environment as though you are defending yourself against saber-toothed cats...it's just the way we are built.
Also...
1--the craziest woman I know has a wallet FULL of family photos of smiling happy faces...including the one taken when her 14-year-old had a friend with a job at...and keys to...the city swimming pool, so that's where the 14-year-old was LIVING when the smiley picture was taken. Photos capture a moment in time...many times a staged, very unrealistic moment, choreographed to hide the chaos that is their reality.
2--Zion....for a while, Utah had the highest porn rental rates in the nation. Meaning, that the happier things look on the surface (see above) the more I suspect that reality is entirely different. This cop didn't FIRE his weapon...he just used it to beat the crap out of his father, the sheriff, who as schtupping the shooter's wife. In Zion.
http://www.sltrib.com/sltrib/news/56697492-78/brewer-police-moab-wife.html.csp
3--I have a lot of experience interacting with cops. I worked in that field (but not with a badge and gun) and I had MANY family members in that field. They are particularly at risk for that behavior. Their professional world TENDS TO BE very black or white...and real life doesn't always conform to that pattern. Cops are taught ESCALATING ways to deal with people who don't do what the cop tells them to do. That may be the simple explanation for the popcorn-armed father vs firearm-carrying retired cop in the theater last week...result, one dead dad.
4--and it's nothing new...forty years ago, a county sheriff's deputy where we lived came home early (planned or unplanned) and encountered wife with boyfriend. He shot them both and took off, in his cop car. They talked the guy down because they had contact with him via his car radio. (Before cell phones.) If it hadn't been for the radio, he may have killed himself as well. It is, too often, how cops solve problems.
5--a slightly smaller version your local situation was played out a year ago, down the street from a friend of our daughter:
http://www.jrn.com/ktnv/news/188086131.html?lc=Tablet
The underlying issue, as I see it, is that when you live in a world that you think is as "orderly" as Pleasantville, and then things start to change...and your adrenalin starts pumping, again, you probably don't have a lot of coping mechanisms at your disposal.