Who was John Thornton and what advice did he give?

John is the man who rescues Buck from Hal's cruelty. He is by far the most admirable human character in the story, kind but full of the uncompromising strength that is necessary to survive on the trail. John is the ideal master because his nature is so dog-like.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.