Why didn't John Thornton persist in discouraging Hal and Charles to continue? What evidence was there that he had sympathy for the dogs?

They all stagger into John Thornton's camp for a rest. John warns the travelers that the ice is thin, and the trail is likely to drop out at any moment. Thornton cannot say too much because they are not his dogs. Hal refuses to listen, and calls the team to order. Thornton mornfully watches the dogs being abused. Only the merciless lashing of his whip can make any of the dogs rise. But Buck remains on the ground. He refuses to move. Hal takes up the club, but Buck will not move. He is too numb. Suddenly John Thornton springs upon Hal and drives him back, telling him that he will kill him if he hits the dog one more time