I am so sorry this happened Shari. Of course you didn't mean the cat harm. And just b/c you made a flippant comment doesn't mean you meant the cat to die or knew it was going to happen. You did what you thought was giving the cat a chance.
I have to wonder about the shelter's definition of feral if he came into your yard like that. He may have been sick when you took him, it is really hard to tell.
Our local Humane Society (run by a total wackjob btw) claims that no shelter regardless of what they say is ever no kill b/c animals come in sick and dying and they have no choice but to put them to sleep. And if the shelter claims that it is no kill - then they must send the animal to another shelter to be put to sleep. That's the wackjob's take on it of course.
hugs to you -
and don't be a stranger - I miss Sammy and Dudley!