Karenin is cowardly and cold. More than that, he stands in the way of happiness. You could argue that the Anna/Vronsky relationship was doomed to fail, regardless of whether they got married or not, but the fact of the matter is that Karenin deliberately withheld a divorce after he had already said Anna could get a divorce. That's just not very nice. So even though he does nice things like forgive Anna and adopt her kid, and even though we occasionally feel sorry for him, it's just not enough for him to avoid the antagonist label.