It isn't his fault; he is a dog and dogs come from wolves, and wolves hunt. Just because dogs are domesticated doesn't mean they don't have some wild insincts left over. Dakota has a high prey drive, obviously, and that is no one's fault. Daisy has a high prey drive too but luckily she isn't fast enough to catch anything. However, my neighbors used to have chickens and sometimes they would just let them go wherever they wanted. They'd come to my house and eat our gardens. My mother hated them! And one day when they wandered over, Daisy got two of them. I don't think she really meant to hurt them ; my cousin saw it happen and said she was just trying to play, but played too rough. Both chickens had their necks snapped. It's sad, but not my fault, nor Daisy's. If people want to let their pets or animals wander around, it is their fault if something happens to them. Since you said it was a stray I guess it really is no one's fault. Don't punish Dakota for being a dog, but I yelled at Daisy for killing the chickens anyway. Just scold him. There's not really much else you can do. I am sorry this is happening, I know it's hard. Good luck!