Thanks for the replies! FWIW I live adjacent to Ellensburg, WA. In other words outside of the city limits although that is really moot as far as Washington State law is concerned. Our laws are very clear that predators may be shot when they are harassing livestock which specifically includes poultry. Once a dog attacks a chicken the dogs owner is required by law to keep the dog contained within his property and our state law clearly gives the farmer/ rancher license to kill the dog if it is caught at large.
I did not notify law enforcement because dogs killing chickens is a civil matter. There also is no duty for me to do so to enforce my rights in the future if I have to kill my neighbors dog.
FWIW I have a background in Law Enforcement and am good friends with many in law enforcement in my area. The county prosecutor is also a friend and his advice is to kill dogs attacking livestock. Some counties have a specific exemption for discharging a firearm in a "no shooting zone" or city limits for protecting livestock including poultry. It is wise to learn the laws of your local area and how those in charge of enforcing them do so.