We've lost many chickens (15?), 2 geese, and 3 goats to several dog attacks. One was our neighbour's dog, the others were holiday visitors animals that they just leave to roam. The other goats were also severely injured, and when the vet came he said the only option is to cull the dogs, as once they have the taste of fresh blood, they will NEVER stop hunting at any opportunity.
The neighbour protested, and fitted a shock collar. The dog came back to our place, thankfully we were here and prevented any further killings. It went on to kill several sheep and a calf on another neighbour's land. Finally, after a year of legal wrangling between the marie and the owner, the dog was destroyed.
We know of sheep farmers that have had sheep dogs for many years operating successfully as working dogs, then out of the blue they will kill a sheep. The farmers will usually destroy the dog straight away, though we know of one farmer who pulled out all the dogs teeth as he said she was too good a sheep dog to kill.
I know many people have successfully kept dogs with prey animals, but unfortunately that seems to be the exception rather than the rule. The vast majority of dog owners don't have the time, understanding, or inclination to maintain proper control of their animals at all times. Regardless of your past experience with an individual, you can never rule out the dog turning on a prey animal one day. Constant vigilance is required.