I am so sorry for such a tragic and needless loss!!!! How could they do that???? There was a similar incident with a shelter here not too long a go (a few yts ago) when neighbours caring for a cat allowed the kitty to roam and s/jhe was picked up, not claimed and put to death. I loathe =kill shelters!!!
If it were me,. I would be seeking legal recourse! It won't bring your cat back but it might make this shelter think again?
"There are two means of refuge from the miseries of life: music and cats" Albert Schweitzer