Poor Sam and poor you. That is so scary, I've had many off leash dogs come running up to me when I'm walking my dogs and I'm always afraid something like that will happen. Years ago we had a pit bull get loose at my work and attack another dog. There was no provocation at all, he went for the throat and tried to kill the other dog who was not even fighting back. I have friends with pitties and they are sweet with people but all are dog aggressive; not entirely, they get along with certain dogs, but they have to be careful what dogs they let around them. So sorry to hear this happened to Sam, hope he'll have a full recovery.