OOOOOH, WOW! That's a toughie! Well, the local humane society by me takes care of wild animals, so I would most likely try to take it there. BUT, if that wouldn't work out for some reason I think I would gas it until it passed away.

No matter how you put it to sleep, it is at peace now suffering no pain...don't beat yourself up over it, at least you did something to help it out and didn't let it go to suffer and be eaten by something....