To start with, DiCaprio's Calvin Candie was utterly villainous himself, but he's your standard "petulant child emperor" character. Like Commodus from Gladiator he is bored with his existence and only cruelty and domineering over others brings him pleasure. This kind of sadism, while utterly villainous, is a different flavor from Stephen's twisted villainy.
Stephen was utterly villainous for the following reasons:
One, he was a conniving, plotting, Shylock-like weasel of a simpering villain. Devising and sneaking around doing his evil masters bidding. The only character that comes to mind for true comparison is Shakespearian courtier Iago from Othello, or Iago the parrot from Alladin for the pop culture referral.
Two, he is a manipulative villain, always putting ideas of cruelty in Candie's head. When Candie admonishes him or dismisses his intentions, he makes them even sweeter with his devil's tongue to the point that he has become the master of this household of horror.
Three, and this is most prescient, he is a traitor. A traitor to his race. Candie was evil, but his evil was aligned with the one systemic norm of the time, racism. I don't believe Stephen even identifies himself as black anymore. He goes so far as to maintain the brutal household's financial affairs, to personally oversee the torture of his fellow slaves in far crueler methods and durations than his master, and upon identifying opportunities overlooked by his master to practice cruelty upon his brothers and sisters, he happily guides Candie's hand to the literal whip.
Lastly, he's a player hater, plain and simple, through and through. Taking joy in the cutting down of others to or below his level is his sustenance. Take Clifton Powell's Chauncy from Menace II Society. Multiply it times 9,999, you're getting closer to Steven.
So it is with all these villainous traits in mind that we experience the utter jumping-up-and-down-hand-clapping joy in witnessing that side of violence that Tarantino calls "the cathartic fun of violence"
