Hey, is this the first fight on the chicken topic? How exciting!
Seriously, Berrie, I'm sorry about your chicken - I had a sick chicken just after I got my first chickens, took it to the vet who gave it a shot of ABs, it promptly died, they insisted on a PM (which I had to pay for ) in case of avian flu...
I think black/purple combs CAN be a symptom of avian flu, but I'm sure your chicken DIDN'T have that.
As far as chickens go now, I wouldn't ever take them to a vet now - simply because I believe that I now have as much or more knowledge of chicken ailments as they do. I suppose actually I might take them if they needed a specific AB, but I would make sure I knew what to ask for.
I was quite surprised that the vet instantly put your chicken to sleep given that she was eating and drinking - I think I would have been inclined to separate her from the flock, give her some 'pick me up' food (high protein - beef cat food, tuna, hard boiled eggs, poultry spice), check for lice/mites, worm her etc.
I have heard about chickens hiding that they are ill too - I think that if the flock suspect a sick chicken then they may pick on it, so they hide it until they think they are dying, and then go into the 'hunched, eyes shut, tail down' position. Certainly what mine do. Sometimes they're not dying, and I bet they feel stupid when they realise that!