What makes O’Brien an interesting antagonist in 1984?

O’Brien has many positive qualities that make him an interesting character, even though he is an antagonist. He is intelligent, able to convince Winston that he is a member of the Brotherhood. He is also witty. When Winston greets him in his prison cell, thinking he is a fellow prisoner, by saying, “They’ve got you too!” O’Brien responds,”They got me a long time ago.” He is also attractive; even after he has tortured Winston, Winston thinks he has “never loved him so deeply” and that “O’Brien was a person who could be talked to.” Finally, O’Brien is mysterious. Does “They got me a long time ago” mean that he was once a rebel, like Winston? Or does it mean that he has been a faithful Party member all his life? The latter seems more likely given the fate of dissidents, but readers cannot be entirely sure.