Tom Chaney, a.k.a. Theron Chelmsford, murders Frank Ross, Mattie's father, when Frank tries from getting into drunken trouble over a gambling disagreement. Leave it to Rooster for the moral ambiguity: Chaney has a black mark on his face and—symbolically—on his soul. Early on, we find out that Chaney "got a black mark in Louisiana when a man shot a pistol in his face and the powder got under the skin" (2.30). Here, Portis is giving a shout-out to early Westerns, which usually told a pretty straightforward good-vs.-evil narrative.
And Chaney does seem pretty evil. We don't actually meet him until the last chapter of the novel, just in time to see him shot twice by Mattie and then eventually finished off by Rooster. Still, that's plenty. Twenty-five-year-old Chaney is just plain nasty. He manhandles Mattie and would easily have killed her if he hadn't been stopped.
At the same time, he might be less of a criminal than Rooster is. Problem is, he totally lacks charisma and is just a big fat whiner whose refrain is, "Everything is against me" (7.136). So we have to ask: what makes Chaney so mean?
According to Mattie, drinking. She says that Chaney "was a whiskey drinker … It led to killing in the end" (6.144). But take another look at his boo-hoo "everything is against me" complaint. To us, that sounds just about the opposite of the book's moral code of being gritty above all else.
In other words, maybe Chaney's real problem is that he doesn't have true grit.