The previous posts were accurate. I would only like to echo that the death of Thornton helps to sever the bonds between Buck and humanity. In his own mind, there had been an ongoing civil war between trust and mistrust of humans. Buck had experienced both extremes, from loving masters to those who embodied cruelty. One of the most rich elements of the novel is the idea that there is an insistent battle between good and evil, trust and mistrust, and that all creatures are the result of this equation. The elements that are placed on one side of it are balanced out by another and the result is simply the consequence of what is placed on each side. In Buck's case, the addition of Thornton's death placed more elements on the side of mistrust, which is why Buck no longer trusts humans. Yet, in the most poignant of acts, he still visits Thornton's grave to mark the presence of the last bond Buck has to humans and their world.
Thornton and his partners are surprised by a group of Yeehat Indians and murdered. Buck comes upon the bodies of his master and friends and, realizing what happened, decides to track the killers. Buck surprises them, killing a number and scattering the rest. His hatred for the Indians keeps him hot on their trail, and he eventually becomes a legend to the tribe as the dog who hunts then relentlessly. The death of Thornton frees Buck at last from human control, and he is at last able to pursue the long heard call of the wild. He joins a pack of wild wolves and presumably spends the rest of his days in the Yukon free from the control of humans.
There are two major ways in which the death of John Thornton affects Buck.
First of all, it breaks the last connection that Buck had with the human world. After Thornton dies, Buck stops trying to fight against the call of the wild. Instead, he gives in and becomes the leader of the pack of wolves.
Second, it makes Buck hate the Yeehat Indians. Because of this, the tribe becomes very afraid of Buck and the pack. Buck goes down in legend as the Ghost Dog because of the way he terrorizes the Yeehats.