I agree with the dog idea. If they come into your yard and your dog gets it then it isn't your fault. You can also take them a few at a time to your local animal shelter and say you keep finding feral chickens in your yard. That way there is no evidence you killed them and if he calls the police and says you did, you can have the police look around if you want. As far as you're concerned a predator got them. And you are not obligated to tell anyone where you took them.