I have to agree with GAchick and Birdmomma.
IMHO, there is no such thing as 100% security unless you keep your chickens locked up (and even that is not always foolproof!) If you are going to free range, you take the chance of something happening, whether it be a neighbor's dog, a hawk, a fox, or whatever. These things can happen even when you are supervising them, but more so when you are not. I don't understand how you can make the statement that they never leave your property. Chickens might have a territory that they are used to, but they are chickens. There's no guarantee of their behavior. Even children will leave a yard if enticed (scary thought.) I'm not putting the blame on any one person, just stating the fact that things happen... it's how we deal with them that makes the difference.
I would also like to state that my yard is completely fenced in. We make every effort to fortify the bottom of the fence with logs and boulders. Despite this, my dog, a labbie-mix will on occasion (maybe a few times a year) dig under the fence and get loose. She runs around the neighborhood visiting everyone and then comes home to our front steps. I hate to think that someone would shoot her if she's found in their yard, but I know that they have every right to do it. She's still MY responsibility. Yes, she gets along well with our chickens, and even acts like a little mother hen around them. I have no idea how she behaves when she's loose, nor will I assume that she's a perfect angel. Most of our neighbor know her and will try to coax her over so that they can bring her home. That's their choice. If I saw a strange animal attacking my chickens, I would not hesitate to shoot it. If it was a dog that I know, I might try to scare it off first.
Sorry to hear about your loss and hope that things are resolved smoothly.