I kind of find it odd that you were given Bactrim for her wound. Don't get me wrong, it is a good antibiotic but Cephalexin or Augmentin might have been better. I always hate to question a vet though.
You say it looks good now that it has fallen off? Why are you considering euthanasia? Because she is slowing? If you are thinking of doing it to relievers of possible pain, know that slowing does not mean pain in every case. Typically animals will guard painful areas or not want to be as affectionate. They may may audible squeaks, they may not. Refusal to eat and drink is late sign/severe pain.
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