I hate to say it but what moron brings a gun to a dog park and then uses it at the first sign of trouble on another dog?
A Moron who knows that dog owners do not always control their vicious dogs, in fact many dog owners of vicious dogs refuse to admit their dogs have problems and bring them to parks and other public places where they may attack other dogs and people. Bringing a gun in only prudent, I wouldn't go to a dog park without one, or at least pepper spray or Mace. From the article, when you read around the prejudice of the terrier owner, it seems to me the guy was justified in shooting the dog in order to save his dog.