If it was on her property it is feral, that it belonged to someone or not is irrelevant. They should have ensured that it stayed on their property in that case.
She wouldn't have known who the cat belonged to, just that it was hanging around making a nuisance of itself. In a rural area it isn't surprising that it got shot.
Maybe it was killing birds she was feeding in her back yard for all we know. My brother has a bunch of bunnies in his back garden, and the neighborhood cats routinely take the babies out. They find the left over bits in their garden in the morning.
I doubt that she has committed any crime, just chosen an unusual method to kill the cat and being inappropriately public about it. It looks like she got it with a clean shot, so there is no question of cruelty to animals or anything like that.
Irrelevant? People still let their cats out, especially in the country. And no, you can't legally shoot them even if you think they are feral. And if you actually take a look at the photo, the cat might not even be dead yet, but rather dying.