I know just how you feel because a most beautiful and friendly tabby cat turned up here last year, I took him in, looked after him, fed him up, paid to have him neutered, and some speeding driver knocked him down and killed him on the country road where I live and never even bothered to come and ask at our house if it was our DEAD cat that I had to collect from the road! Perhaps not as bad as your cat's suffering though. How can anybody be so inhumane?
OK so the cat was risking it on the road but he didn't usually go up there, plenty of fields to wander about in, but what can you do?
We still miss this friendly, sparky cat although we didn't have him very long.