Any time this decision is made over a pet that we have stewardship over; these questions will arise. It's good that they do, because if it were an easy, guilt free decision I think that it would be overused. After all, it's easier to buy a new $6 chicken (hamster/rat/whatever) than to pay a $100 veterinarian bill. Also the difficulty of ill animals is their obvious lack of communication and understanding of letting us know exactly what's wrong, or being tolerant of treatment. Since I've started with chickens, illnesses are daunting - so many illnesses that need different treatments, show nearly identical signatures in the ill or injured bird. Sometimes it only takes choosing to treat one of them instead of another to allow the disease to devastate the animal and often treating multiple effects at the same time isn't really possible because of the amount of meds needed in such a small animal. Then there's the "this disease causes the following symptoms (a,b,c,...) or none at all...wth?
As for euthanizing the bird; it was probably the only real decision. If it were something contagious, then you could end up with a devastated flock if you allowed the bird to remain. This was my friend's biggest obstacle with his new silkie. They'd only had in a little over a week, but still invested a lot of time in the nursing of it. Not so much attachment as admiration of the animal as it tried to do things it simple could not. If yours was like theirs; the animal seemed healthy and alert; except for not being able to walk and/or balance, so putting something down that still "seems" like it has a chance is tough. But the longer they waited, the less it walked and the skinnier it got. It became a bit more vocal and humans always try to figure out what they're "saying" so it isn't an emotionally 'easy' process. But the fear of having that happen to another bird, or dozen birds, or whatever isn't a welcoming situation, so while the situation is sad, I think it's also easily explainable. My friend was worried of Marek's (and all the pathology tests are still out) but came back with coccidiosis which can just as easily wipe out a population, so even if Marek's comes back clean; a heavy dose of coccidiosis (or any potential pathogen) justifies a tough decision I think...
Sorry about your bird and with a few days of interacting with your healthy and happy remaining birds I hope to find that place where you can wrap your mind around the whole ordeal and realize that while not pleasant, the decision was likely the best (maybe only) decision to make.