Too often people with "issues" tend to migrate into law enforcement.
People with a chip on their shoulder, an axe to grind, some temper problem they look at law enforcement to act out. And most likely they also have a twisted love affair for guns.
So.... why not try the police force?
Local governments have a hard time finding qualified cops. It is a dangerous job. So these guys slip by and get hired that should have never been hired in the first place.
I think that is why they are fired so fast when they screw up.
For the bad cops, the department probably had their eye on them.
Was already aware of problems with the cop. Word gets around.
I doubt this type of thing comes out of nowhere. Or really surprised anyone else on the force.
But until some screwup like this happens, the bad cop got by and never faced the music.
Then, one day the shit hits the fan.
I guess this is the nature of law enforcement.
How can the department find qualified cops? Find good cops? Find people willing to risk their life that aren't already a little wack-o in the brain?
But at least when this kind of thing happens, the bad cop is removed fairly swiftly from employment.