You have to remember, that doing this safeguards your chickens and your kids. This is never easy to do, but is necessary. With an air rifle it will be hard to accomplish 0ne shot, one kill. If this ever happens again, try shooting him directly in the ear, this should end his suffering...
I have 2 Border Collies, I trust my dogs with my chickens and also trust them with with the security of my property. My dogs are very trust worthy and follow my every command. If you don't have Border Collies, then you have no real idea what your talking about...