The owner was acting responsibly; the dogs were contained, until they, enraged by an canine intruder, pulled free by slipping a collar and breaking a chain.
This was a matter for catch poles, and perhaps a fine delivered to the owner for failure to anticipate the growing strength of her dogs and provide a tie-up strong enough to keep a fully grown mastiff contained. Not a shooting.
As has already been mentioned, having a fence on one’s property is legally impossible in some areas of the country. You would deprive Freepers living in those places of the excellent first alert service that a good watch dog provides when the criminal element (or bad cops intruding on private property without just cause) come lurking?
The dogs got loose ergo the owner acted irresponsibly.
It is cruel to chain a dog.