That may be the case, but to be able to determine what the problem is, you MUST do a necropsy.
In my mind, it is better to be safe than sorry...
If an animal like that was reacting that way, I would euthanize immediately to prevent any possible spreading of anything, and to prevent the animal from suffering any longer.
Some may not agree, and that is fine.. but I don't see the point of prolonging the suffering of that animal, as it already looks like it is on deaths door.