However, if this dog has shown signs of aggression in the past to humans or dogs, which I'm sure it has at some point, it should not have been allowed to roam freely even on the owners property. A responsible pet owner would take into consideration the possibility of a person wondering on the property. Whether it be a neighbor child, etc... and not take that chance with their animal. Even if it had shown no previous signs of aggression to people, chances are it has shown them to other dogs, and was still allowed the possibility of injury or death to itself, should another dog encounter it. It should have been restricted to the eyes of the owner at all time, if the owner was concerned for the well being of the dog.
Accidents do happen, dogs do get off leash, gate get left open etc, but this states that the pit was granted free reign to the yard all time. so it was clearly not under the watchful eyes of the owner. The only thing that could give me a moments pause is if the yard was fenced. But then that comes full circle to how did the other dogs get in. If there was no form of fencing, electric fencing, tie out, etc, then in my opinion that puts the owner smack dab in the middle of irresponsibility in my book. I mean you can't just assume in good conscious that your pet will just stay in your yard at all times unwatched.
The two dogs that were killed were defientely not strays, as it states they belonged to neighbors. Off leash and allowed to wander, yes it appears so, owners neglectful, if they did wander off leash? Yes. But that puts them in the same boat as the pit owner, dog off leash, no control to keep the dog safe. I come back to my point, you can't assume your dog will stay in your yard safe, if there is no restraint. Maybe just my opinion on it though.