The bad guys are pretty obvious here. Just find the ones rampantly abusing White Fang—twisting his soul like so much silly putty—and you have them. Beauty Smith makes the obvious choice, as does Lip-lip. The former turns White Fang into a killing machine, then sets him loose for fun and profit. The latter just makes his early life a living hell, like a lot of bullies do.
But underneath them lies a much bigger antagonist: the Wild itself. As London says, "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). Beauty and Lip-lip may just be tools of the Wild, trying to kill White Fang. Naturally, he fights back… making the Wild the biggest, baddest antagonist of them all.