Monks is the primary villain of the novel, in that he’s the one who’s really out to get Oliver, but because he appears in so few scenes, he’s listed lower than some of the arguably more "minor" characters.
George Gissing, another Victorian novelist, argued that one "blemish" of Oliver Twist was "Monks, with his insufferable (often ludicrous) rant, and his absurd machinations" (source). Monks’s "machinations" do certainly play a big role in the plot of Oliver Twist – after all, it’s Monks’s "machinations" that move the whole plot forward.
Why Monks's machinations "absurd"? Well, Monks goes after Oliver for no reason other than pure malice. After all, Oliver’s got no way of knowing who his parents were, and even if he figured it out, he would have no way of knowing that there was a will that would have left him some money. Monks’s mother had already burnt the will, and had attempted to ruin Oliver’s mother’s little sister (Rose) for no reason other than spite. Sure, Monks’s father’s attempt to cut him out of the will might have hurt Monks’s feelings, but once the will was burnt and he’d collected all the money, what’s the point in going after Oliver? Why not just leave him to rot in the workhouse, or die in the clutches of Fagin like so many other boys? OK, so maybe "absurd" isn’t a bad way to describe Monks’s "machinations."
Gissing’s other criticism is that Monks rants too much. Maybe he’s right – Monks does tend to go on a bit, and when he does, he works himself up into a frenzy.
An interesting thing to note about Monks’s frenzies is that he’s not just wild with hatred – when he has "fits," it’s not because he’s completely lost his self-control. Although Dickens never names the disease or disorder that causes those fits, most critics believe that Dickens intended to describe symptoms of epilepsy. His frequent "fits" have, over time, left an imprint on his face, and not just from biting his lips. Mr. Brownlow says that Monks’s "evil passions, vice, and profligacy festered till they found a vent in a hideous disease which has made your face an index even to your mind" (49.57). So Monks’s physical disease has made his face as distorted and ugly as his vice and crimes have made his mind. Hmm. Funny how in this novel, that’s actually true for most characters – their outsides match their insides. You can tell who’s a good guy by how he or she looks.