We mention in "Symbolism, Imagery, Allegory" (under "Heraldry") that Front-de-Boeuf means Ox Face or, even more literally, Ox Forehead in French. Now we've seen more than our fair share of cow foreheads (and oxen are just cows, really), and we think they're quite nice: broad, calm, not particularly mean-looking. Of course, we know that bulls can be violent – we've seen footage of bullfights in Spain. But even so, we don't think of cow foreheads as particularly wicked.
The face of Reginald Front-de-Boeuf, on the other hand, is vicious, fierce, and "malignant" (22.5), or filled with evil. The only thing Front-de-Boeuf really has in common with his namesake is his great height and large muscles: this guy is plenty big and strong. (You know the saying "strong as an ox"? That's Front-de-Boeuf.) Front-de-Boeuf uses his physical bulk to intimidate people. His face is also scarred with the marks of past battles, which helps prove that he's a bad guy, bloodthirsty and quick to anger.
If Front-de-Boeuf's appearance isn't enough to convince us of his evil character, his deeds are. He tries to torture Isaac to force him to give up his fortune. Everyone in the neighborhood hates him for his cruelty, so much so that the outlaws jump at the chance to invade his castle at Torquilstone and burn it to the ground. And we also find out, according to Ulrica's confession, that he killed his own father in a fit of rage.
It's pretty clear that Scott believes in something like karma. As Front-de-Boeuf lies dying while his castle burns around him, he seems terrified at the idea that he's going to hell. Front-de-Boeuf has sinned freely throughout his life, and Ulrica reminds him that he'll have to pay for his crimes in the afterlife. Her gloating only increases Front-de-Boeuf's horrible deathbed anxiety. By suffering through such a painful and angsty death, Front-de-Boeuf gets his just desserts according to the moral code of the novel.