Sorry folks I am sticking to my guns, dogs that Pursue and Kill their chicken prey (as opposed to just picking it up and accidentally biting down too hard) have an instinct or a love of killing that is almost impossible to eradicate. This dog deliberately planned an escape with the purpose of attacking those chickens. You can certainly wait years and hope things will change, but that is kind of experimental and you may suffer the losses of not just your chickens but other animals as well. I do not think those dogs should be killed though, there are many homes that they would be perfect pets for - with no chickens around. Their are probably trainers that could improve the situation, but those dogs are never completely trustworthy around chickens again.