Yeah, I can't excuse something like this for any reason. If a dog is threatening you, and for some strange reason you just happen to be putzing around the yard with a machete, then you do what you need to to get it out of your yard, you don't hack up an animal on the run. I totally get people shooting a dog that is threatening themselves, their family, or their animals. It sucks, but I can see where it might be necessary to protect you and yours.
I have used a machete before in our yard on our ivy, and I'm not a weakling, but it takes energy to use. It would take work to hack a dog to death with one, while chasing it to boot. It would also take a special kind of person to continue to injure the animal as it fled.
I guess it also comes down to the kind of community you want to live in. I do NOT want to live in a community that has tons of loose dogs, but what I want to live in even less is a community where this sort of thing is allowed to slide.
Yes, the owner should NEVER have let his dog run loose. However, that does not make what was done to the dog acceptable course of action for anything short of an actual attack.