This is not meant to be a comment on the specific circumstance of this thread, but more to the "no such thing as accident" sentiment that has been expressed.
The plain and simple fact is that people make mistakes. They make small mistakes, they make big mistakes. The very nature of people is that they are not perfect. People see things where there isn't anything to see. People don't see things when they are right in front of their faces. The mind is not infallible in any aspect of what it does. This is something we accept as being human. Otherwise humans won't be allowed to do anything. To let people do means to let people fail.
Mistake != Accident
And when you make a mistake WITH A GUN, it's negligence. Especially if you shoot a PERSON while "trying" to shoot at a dog, neither of which is legal.
The 12 Golden rules for Safe Gun Handling
1. Always treat the gun as loaded.
2. Always keep the gun pointed in a safe direction.
3. Always keep your finger straight and off the trigger until you are ready to shoot.
4. Always keep the gun unloaded until you are ready to use it.
5. Never point the gun at anything you don't intend to destroy.
6. Be sure of your target and what is beyond it.
7. Learn the mechanical and handling characteristics of the gun you are using.
8. Always use proper Ammunition.
9. Be sure the barrel is clear of obstructions before loading and shooting.
10. If your gun fails to fire when the trigger is pulled, hold your shooting position for several seconds; then with the muzzle pointed in a safe direction, carefully unload the gun.
11. Don't rely on the gun's safety to keep it from firing.
12. Be aware of your surroundings when handling guns so you don't trip or lose your balance and accidentally point and/or fire the gun at anyone or anything.
Range Safety
1. Follow the 12 golden rules.
2. Know and follow all the rules of the Shooting Range.
3. Listen and do what the Range Master tells you to do.
4. Un-case and case your gun at the shooting bench, never behind the safety line.
5. Always keep the barrel pointed down range.
6. Always keep the gun on safe until you intend to shoot.
7. Always wear eye and ear protection when shooting.
8. Never shoot at water or hard surfaces.
Hunting Safety
1. Follow the 12 golden rules.
2. When hunting in a group, always pick one person to act as a Safety Officer for the Day or Trip.
3. Establish and share everyone's zone of fire with each other and know where everyone is at all times.
4. Always keep the gun on safe until you intend to shoot.
5. Never climb over anything with a loaded gun in your hand or on your person.
6. Never use a scope on a gun as Binoculars.
7. If you fall or trip, control your muzzle. Afterward, check the gun for damage and/or obstructions in the barrel.
8. When in Doubt; Don't shoot.
By my count, this "officer of the law" (state trooper) broke several of those "rules" never mind the LAW.
When you break that many rules all at once, the result is a foregone conclusion, not an accident.
I don't see many of the brothers of the thin blue line defending him. But neither do I hear any outrage. Their silence tells me all I need to know.