You have utterly failed to explain how a dog on it's owners property was a threat.
If an unconfined (or invisibly-confined) dog attacks, expect the target to defend himself ... regardless of where the dog is in relation to property lines.
For instance, I’d sooner shoot an invisibly-confined pit bull on your property than let it get within 10-feet of my five-year-old daughter. I don’t know that I’d have shot in this case — but I don’t know that I wouldn’t have, either.
SnakeDoc