Hey there, just wanted to let you guys know that I did follow through after what you said, though I got results I didn't predict.
Though I was hesitant at first to call the county animal authorities because I felt I was signing away the animal's life, as time passed he was getting really bad off. But the county actually surprised me by referring me to a wildlife rehabilitator, who gave me medicine to treat the fox's sarcoptic mange. All I had to do was slip it into some chicken legs and leave it out for him.
The sad thing is, I hadn't seen him for a while before I called the authorities, and although the chicken disappears every day there are no longer any fox markings nearby. Even if I had tried to trap him myself and have him destroyed, it was probably too late. So despite the effort, I think he is dead already.
But the good news is that the county was very supportive of the local wildlife, something I hadn't expected, and the experience inspired a new painting on my website.