I am curious about something. The family was gone and the dogs were only contained by an electric fence? In the front yard? Or was somebody home at the time of the incedent?
Not that will make a difference because your dog was not under control and killed the little dog, but I guess I question why anyone would leave their dogs, even with an electric fence and collars on the dogs, alone and basically loose in the front yard?
What if a stray dog had come into the yard, crossing over the electric fence and killed the dog or dogs?
It just struck me as odd that anyone would leave their dogs unattended in the front of the house even with collars on. That would not stop a stray from doing exactly what your dog did, you fessed up and are facing the uncertain future, they left their "expensive" breeding dogs pretty much uncontrolled in an unfenced yard.(unless I misunderstood what I read)
Whatever the circumstances, I am sorry this happened to you and your dog, you might be facing a long battle.