So sorry, Mary! I know you feel bad, but we're just human, and make mistakes.
I had the same thing happen this week.
Yesterday, after I got home from an early work day, I let the hens out of the coop. All was going well as I used the lawn tractor and cart to clear and haul away all of the coop litter. The dogs were elsewhere.
About an hour after I was done, I see that the devil dog is playing with a hen, on the other side of the house, a good 200 feet from the coop. Hen was playing 'possum and the pup was just barking at her. Recovered the hen, checked her for wounds and injury and she appeared fine. I took her back to the coop and put her in a nest box.
Half an hour later, both dogs were tussling over a different hen. She was missing a lot of feathers and had a bloody neck. Gathered her up and put her back home. I thought that is either was fatally wounded it was her.
Checked the coop this morning and the first hen was dead. I would have figured that the more obviously wounded hen would die, but it was the first one. I have failed in my stewardship. So I know how you feel Mary.
Today, I found out how the hens were getting out of the run. I saw a hen leap up to the wood pile and then over the 6 foot fence. The pup also saw that and the chase was on. At least I was there to prevent any harm.
So now I have two chores. One is to complete the top netting. The other is to remove the wood pile that they are climbing. I feel just awful about what happened. I have to get better at this living in the woods thing. Damn, I feel bad.