Saw that one floating around a while. While I hate the thought of a dog being killed, when it's in self defense or in defense of one's child, nothing is off limits. It's sad that it had to come to that but the blame, as usual, lies with the dog owner. Didn't know the dog was loose? All the dog's shots are up to date but she doesn't know about rabies?
I'm also skeptical of the “never acted this way before”. You always hear that line. I even once heard “he has never bitten anyone before” from a dog that roughly a year earlier had bitten me.