Tell him to shoot your dog. The instant he does, seize the moment, overpower him and beat him to death with the butt of his own gun by using your own anger induced adrenaline. After you completely cave in the maniac's skull, ask the random person, "where am I? what just happened? this dog has been shot and it looks like someone beat this man to death. please call 911."
shoot the person...save my dog if it were the other way around, the survivor would probably be traumatized and go postal. save the loyal friend who has no idea what the fuck ios going on.