If Tom is the book's Christ-figure and George Harris its revolutionary Romantic hero, Simon Legree is without a doubt its anti-Christ, its arch-villain, or — as Cassy describes him — its vampire. If Tom's bearing and behavior show that he is among the elect, Legree's show quite definitely that he is not.
In several ways, as a character, Legree is indeed Tom's antithesis. We know nothing of Tom's past except that, as Shelby tells Haley in the first scene, he "got religion" at a camp meeting four years before the book opens. Of Legree we are told that, after he had spent some years at sea living a dissolute life, he was "almost persuaded" by his mother's prayers to reform but instead chose sin. (In both cases, according to traditional Calvinist doctrine, the apparent choice was really only an outward sign of the condition of the men's souls; however, Stowe's narrator describes Legree's critical moment as a genuine conflict between good and evil in which evil triumphs — just as, in Tom's moment of near despair, love and hope win the "victory.")
Now, whereas everyone whose life Tom touches is lifted and helped, Legree affects everyone near him for the worse. He has no family, only the artificial and perverse "family" he forces his slaves to enact: Cassy his "wife," whom he has used until nearly all of her actions (except those inspired by Tom) are hateful reactions against Legree; Emmeline his "daughter" whom he stole from her own mother and now wishes to force into an incestuous relationship (the nature of which Cassy senses, in her protection of the girl); Sambo and Quimbo his "brothers" (or "sons"), whom he uses as companions and henchmen, alternately punishes and rewards, and has turned into tools for draining the life and dignity from the field workers.
Like all of Tom's owners and like the book's other profiteers from the business of slavery, Legree is a materialist who sees human beings as nothing but material that can be used for profit. Like them all, with his reduction of slaves to the status of things, he has necessarily reduced himself to the same status — for part of what this book teaches is that to objectify others is to objectify oneself. But whereas Haley — probably genuinely — sees the nature of what he does only dimly (he has lied to himself successfully for so long that he believes he truly is a "man of humanity"), and whereas both Augustine St. Clare and Tom Loker, in different ways, are still capable of the "change of heart" that might save them (although in St. Clare's case it saves no one but him), Legree not only sees clearly what he is and what he does to others but also revels in it. He may tell the men in the riverboat bar that he uses slaves up and buys new ones for economic reasons, and he may pretend to Cassy (and even at times to himself) that he is concerned to keep Tom more or less healthy, or at least alive, in order to realize Tom's cost to him. But in truth, what he really wants is to exercise the absolute power of life and death — and more, the power of moral destruction — over these people.
Just as Legree uses alcohol "cautiously," so he exercises his power over his slaves as cautiously as he can, knowing that his real impulse is to go too far — for he hungers, perhaps even literally, for their blood. He reveals what he truly is in his jealousy of Tom, whose power to give hope and humanity to the other slaves cannot hurt Legree economically. What Legree wants, finally, is worship and fear; he has gone beyond capitalism and the profit motive and come out the other side.