If Chaney is nasty, then Ned Pepper is just pure evil, like a little glimpse into what Chaney could become if he stuck it out for a few more years. Almost everything he does is violent, violent, and more violent. Just look at the way he treats Mattie—who, remember, is a fourteen-year-old girl (albeit one with a gun):
We don't ever get much sense of what motivations lie behind any of Pepper's actions, although we imagine that, by now, his two goals are (1) don't die, and (2) don't get caught. He does have some sort of rudimentary moral code, telling Mattie, "I have never busted a cap on a woman or anybody much under sixteen years but I will do what I have to do" (7.59).
He's no Frank Ross, but at least it's something.