I know what you must be going through after having to have one of my favourites put to sleep last week. She got into the garden after pulling some wire out of the way, wandered around and stood on a rat trap that broke her leg in 2 places. She sqwauked a bit, wasn't trapped and I found her hopping merrily around on one leg. She was at the vets within 20 mins who said it was such a bad break that he couldn't do anything for her. By now she seemed to be going into shock as she was gasping for breath, so I just held her gently whilst she had the injection and I spoke to her, thanking her for the eggs over the last year and saying that she would soon feel easier and off she drifted. I felt guilty about the trap, but it had been well out of the way and they are never allowed in the garden but stay in their large fenced area in a field next to the garden.
However about 4 months ago I had noticed that one chicken seemed a bit subdued as you put it. I decided then just to keep an eye on her during the day. Unfortunately later that day I found her dead in the run, no visible signs of injury or disease.
I felt more upset about that one, thinking that she must have suffered and vowed never to wait and see again and I wouldn't want anyone to have that sort of guilt trip. You made the right decision for your pet by taking her to the vet. Next time, and there will be a next time if you continue to keep chickens, you will know what you must do. Doesn't make it easier for you, but it does for them x.