The Wild is almost a character itself in White Fang. London really dug the wilderness, but he also understood how frightening it was, and how fast it could kill you:
It is not the way of the Wild to like movement. Life is an offence to it, for life is movement; and the Wild aims always to destroy movement. (1.3)
It has rules, usually written in all-caps to better make the point. (London was stunning in his ability to predict Internet trolls.) First, White Fang learns, "EAT OR BE EATEN" (15.8). And later, he must "OPPRESS THE WEAK AND OBEY THE STRONG" (13.11). The Wild's rules dominate White Fang's thoughts, so much that they literally jump out from all the other words. That's how strong the Wild is and how irresistible its lessons can be.
London could be taking several directions with this. If we think of White Fang as a journey away from the Wild and towards civilization, then the Wild is kind of the ultimate villain: something White Fang needs to resist and eventually escape.
On the other hand, London emphasizes the fact that the Wild is pretty dominant, even in man's world. We see its pecking order in people's interactions, like when the men at the dog fight call Scott "one of them crackerjack minin' experts. He's in with all the big bugs" (18.86). Scott may be civilized and well dressed, but he's still an Alpha Dog, isn't he? And if that's the case, that means that there's no real escape from the Wild. It's not a villain, but more of a fact of life, like zits and back pain.
Along those lines, we might choose to think that White Fang isn't trying to escape the Wild so much as make peace with it, and even control it to do the things he needs to in his new life (like savaging intruders with murderous intent). We know he's become civilized by the end of the book, where "There was plenty of food and no work in the Southland, and White Fang lived fat and prosperous and happy" (24.1).
But he's still "different" from the other dogs (24.2), and that difference really pays dividends when Jim Hall comes a calling. White Fang "gave no warning, with no snarl anticipated his own action" (25.15), where a civilized dog would have barked. Afterward, he heals from his bullet wounds because "a constitution of iron and the vitality of the Wild were White Fang's inheritance, and he clung to life, the whole of him and every part of him, in spirit and in flesh, with the tenacity that of old belonged to all creatures" (25.29).
Clearly, he has a toughness given to him by the Wild, mixed with the morals of his nice-guy master. That's the best of both worlds, a balance that helps him find peace. Maybe there's a lesson in there for us, too, to find the best parts of our more unruly side and make them do good things within the confines of civilization.