I am not 100% cetain but I'm pretty sure it says something in the wildlife code about dispatching a sick animal then contacting the PGC. Same as if you are a homeowner an the sick animal is in your garage.
Wetzel says the fox had mange possibly distemper, who are we to question him on that? Certainly it is possible it did have a sickness.
At the very least any 1st yr law student could make a strong case for Wetzel shooting at it to protect himself.