We pulled Declan out of the shelter.  He was filthy and his claws had grown into his pads, leaving big puncture type wounds.  He was unable to use his rear leg.  We took him to the vet and he had cancer that had metastisized in his lungs, and the only humane thing to do for him at that point was to let him go out of this world with dignity and with people around him.  If you have a dog, they deserve their humans to step and do the right thing by them.  That doesn't mean cutting your dog loose when he becomes ill.