I agree with everyone else, karma will sort itself out, and I think you should save that poor kitty!

My parents used to live next door to these people who kept cats but never took care of them. One day a young female got hit by a car. What does the neighbour do? comes knocking on my parents' door and expects my mother to take her to the vet! She did, of course, and paid for all the expenses to boot. The cat was treated and released and they didn't even so much as say thank you!! the poor kitty eventually disappeared

So I think you should keep that kitty!