How was Buck transformed once he passed out of Thornton's sight?

Buck has two identities at this point: one as sled dog in Thornton’s camp, another as wild hunter in the forest. He kills a bear and fishes for salmon the river; when the moose come in the fall, Buck hunts them eagerly. He cuts a bull away from the pack to kill him and finally brings him down after four days. Then he heads back to the camp. On the way, he feels a strange stirring in the wilderness, of something new abroad, and he feels a premonition of calamity. His feeling is proven correct when he finds Thornton’s dog Nig and one of the dogs bought in Dawson, both dying on the trail. As he approaches the camp, he sees Hans lying facedown, arrows covering him. He peers out to where the lodge had been and sees Yeehat Indians dancing in the wreckage. Buck charges, cutting their throats with his fangs and killing several of them. The Indians scatter, and Buck finds the rest of his camp, including Thornton, dead.

Buck mourns his dead master but feels pride at having killed the Yeehats. Henceforth, he will not fear men unless they carry weapons. He hears the call of the wolf again. His ties to Thornton broken by death, he heads off to follow it. He finds the pack, and one wolf lunges for his throat, but he breaks its neck easily. Three others try but pull back. After half an hour they all draw back, and one of them approaches Buck in a friendly manner. Buck recognizes him to be the wolf he encountered in the woods. Buck joins the wolf pack, and the Yeehats notice a difference in the local breed of timber wolves as years pass. They also tell of a Ghost Dog that runs at the front of the pack, singing songs and leaping above his fellows. They tell of a haunted valley—where Thornton lies dead—where an evil spirit dwells, and where, every year, Buck comes and mourns for a time beside the stream before loping away to rejoin the pack.