I'm so sorry for what happened. I know it's selfish, but I wouldn't warn the neighbor about it unless you're confident your neighbor is a responsible person. After all, if you warn them, and later you have to shoot the dog, they will automatically know who's responsible, and if they're crazy they can make your life miserable. I've heard way too many horror stories on this forum. If you can figure out a way to mention to them that a dog killed your chickens, but not necessarily their dog, and how upset about it you are, that might work. Hopefully if it was theirs, they'll be smart enough to connect the dots on their own.