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

When Buck is being whipped Thornton can't take it. John Thornton springs upon Hal and drives him back, telling him that he will kill him if he hits the dog one more time. Hal tries to retaliate and gets his knuckles slashed for his troubles. John cuts Buck's traces, and the team proceeds. A quarter of a mile away, the ice breaks and the sled goes down, taking humans and dogs with it. Thornton's instinct has made him wise in the ways of dogs, but his experience has made him wise in the ways of people. The other dogs know no life but the traces, or they have too much fear of the club. John sees Buck and understands his silent revolt. He saves him, because he recognizes his value. John knows the kind of loyalty and power necessary to survival in this kind of world. When the sled team goes down, the reader may or may not experience pleasure at the death of Hal, Mercedes and Charles, but the death of the dogs is poignant, even heart-breaking. Even more so than the single deaths thus far, this scene drives home the consequences of attempting to live in this world of club and fang.