I currently have a friend with a cat who's had to have half of her tail amputated, and who held her next door neighbour's cat as it died, after a greyhound moved in up the road. The owner doesn't seem to think there is any problem with her dog chasing and injuring / killing cats on the road and in the cats' own gardens.
So I am struggling with the rehoming of a dog that's learned to kill a cat, unless there is some proper and very robust vetting of the prospective new home and owner.