What does John do to the man beating Buck?
In The Call of the Wild, chapter 5, it is clear that Buck's new masters--most notably Hal--are incompetent. They overwork and underfeed their dogs, who grow increasingly weaker. However, Hal and Buck's other owners do not recognize their incompetence, so they assume their troubles stem from their sled dogs' laziness. Consequently, Hal beats Buck and the other dogs to try to make them work harder.
Eventually, the sled team comes to John Thornton's camp. Eventually, the humans decide to leave the camp, but Buck senses danger ahead. Thus, he refuses to leave. Hal beats him, but the torture does not convince Buck to obey. Hal continues to torture the dog; this angers John. John threatens Hal and hits him with his axe handle. The pain causes Hal to drop his knife. John picks the knife up and frees Buck from the sled. John's compassion saves Buck's life, for the ice collapses soon after this event; Buck's former masters, their sled, and the sled dogs all fall into the icy water.