White Fang has several villains, but none are more villainous than Beauty, a sometime cook and dogfight enthusiast who buys up White Fang to battle other dogs to the death in illegal matches. If Scott's the high point of human civilization, then Beauty's the low point—the flip side of White Fang's hunky rescuer. He inflicts torment and pain on White Fang as a way of padding his own pocket, which drives the poor pup mad with rage and sets him loose on other twisted beasts for the amusement of callous onlookers.
In other words, he's as awful as they come. Luckily, he takes a pounding quite well, letting the good half of civilization (i.e., Scott) take White Fang after full exposure to the evil side. It's a nice dynamic and Beauty certainly seems to enjoy being awful, so who are we to question his villainous status?
Also, it's worth noting that the other residents of the fort tolerate Beauty, "in a broad human way, as one tolerates any creature evilly treated in the making." (16.7). London suggests that tolerating an awful person is pretty awful in and of itself, and that the whole community sinks into the slime pool by letting someone like Beauty crawl up out of it.
London also spends a lot of time describing Beauty physically—three paragraphs by our count. Contrast that with Scott, who London just describes as "tall" or Grey Beaver, who's "an Indian." This is not a writer who wastes a lot of words and yet he delves fully into every one of Beauty's hideous physical features.
Why is that so important? Possibly because London wanted to impress just how awful he was. If we're physically repulsed by him, we're probably going to hate his inner self all the more. It's not like he's going to win us over with his charming personality. He's ugly inside and out, in case you didn't get the hint.
What makes him so awful is that he capitalizes on White Fang's inner Wild creature. He takes advantage of White Fang's roots in the wilderness, and makes him even more savage than he would have been if he were just out there fending for himself. In the Wild, White Fang's kill-or-be-killed attitude is all about survival. He has to be that way.
But in the ring, his ferocity is only in the service of dolla dolla billz y'all. And that makes it straight up cruel, rather than just, you know, natural selection at work. We think that's what makes this guy truly deplorable—the fact that he twists natural instincts for profit, when he could have just left well enough alone.