This is another tough one. Sutpen is definitely a villain and a brute. He is the antagonist to everyone else's desires in a sense. But if Sutpen is the protagonist, can be the antagonist, too? Well, sure. He is pretty self-destructive after all.
On the other hand, Sutpen would likely say that the antagonist is Charles Bon, his own son, who fouls up his design by demanding to be acknowledged as part of the family.
Or we can just ignore both of these guys and call the South the antagonist. After all, it's Southern society that poses a problem for Sutpen to begin with, and is the conflict that leads him to start his "design". What do you think? Who (or what) do you find the most antagonistic in this novel?