> I don't particularly adore cats, but whoever did this is about as low a form of life as there is
Agree. I dislike cats, but that's no excuse for this kind of cruelty.
> Take it to a shelter or just turn it loose so it will at least have a chance to fend for itself.
Never "turn it loose" it into the wild to fend for itself. Have it humanely put down instead. Letting a cat into the wild allows it to go "Feral". When it goes Feral it becomes a predator at the near-top of its food-chain, with a devastating effect on wild native animals. Its progeny revert to ancient, nondomesticated cat ways. After a few generations they become quite dangerous.
In NZ and Oz, Feral Cats are a major pest. They probably are, too, in North America, except you may not notice it as much, given your landmass. You may never know how many species of native wildlife you may have lost.
You should never "put down" a perfectly healthy animal just because you don't want it.
There are plenty of options for people who don't want their pets anymore, and no decent vet will agree to put a healthy animal to sleep, ever.
I completely agree with you about "setting it free", though. A domestic animal is not fit for the wild. That is why they are called domestic.
People are starting to notice what feral cats can do to wildilfe in North America as well. Earlier this year, the state of Wisconsin floated an idea to have a hunting season for feral cats (or maybe it was to hire hunters - one or the other), because the cats are decimating local bird populations.
At any rate there was such a storm of public protest that Wisconsin dropped the plan like a hot potato. The media ran story after story about how the state was going to let evil hunters gun down Fluffy in the yard in front of the children. Obviously these people have never had any experience with what nasty critters feral cats really are.