With any pets killed my chickens or my chickens were killed by pets scenario there are two sides of parties that have the responsibility of protection from such an event.
It sounds like you did your part keeping your dog on your property but there was a momentary lapse where he got out, accidents happen. My question is, was there any fence around your neighbors yard to help keep the chicken safe?
To be clear, I would be ******/sad if someone's dog came onto my property and killed one of my chickens. That being said, I have their area fenced and the only way a dog should be able to kill one of my chickens is if it flew over said fence or I left a gate open by accident. Keeping them safe is my responsibility, keeping dogs in your own yard is your responsibility - ideally if one of those two fails the other will not make the same mistake on the same day and there will be no harm done.
What you say is 100% correct. Unfortunately there are a lot of people who don't understand this, or who don't care about others. Leaving people who do care to have to deal with their mess.