I haven't read the whole thread -but- I honestly don't think the owner is responsible for his own dog's death. I also don't believe he was an irresponsible pet owner.
Accidents happen. Pets get out, because someone left the gate open, or the dog learned open the gate (ehem, Gipsy), or the slipped the leash. ALL these things have happened to us. I don't think it makes us irresponible pet owners. I think in the 12 years we've had Gipsy she's gotten out a few times, because she learned to open the gate, or because she can open doors, she slipped the leash ect. When she slipped the leash, we bought a new leash, when she learned to open the door, we locked the door and when she learned to open the gate we put a rope around it so it won't open when you push the handle down. This doesn't make you an irresponsible pet own and it doesn't excuse someone hitting your dog with anything.
Everyone in my parent's neighbourhood keeps their dog off leash - we do too - Gipsy listens to commands and has never wondered off. It could happen though, but really? I would expect it to be common decency to not kill my dog. If you're afraid of a dog you call the police unless he is literally bitting you or a family member and you need to defend yourself. This dog was standing in the yard and barking. That hardly qualifies for needing to defend yourself.
There used to be a dog a few streets down that was an escape artist. Their fence grew higher and higher over the years and still he got out. His owners were pretty responsible - they ALWAYS had him on a leash and never let him out in the yard alone ect. I still brought him home countless times, because he jumper our fence to place with Gipsy. He slipped the leash a few times while I was bringing him home.
Accidents just happen. Dogs & cats get out, kids run off in supermarkets, people forget to shut gates.