In the film A Doll's House, directed by Patrick Garland in 1973 and based on the 1879 play by Henrik Ibsen, the main antagonist seems to be Krogstad (Denholm Elliot). I, however, believe that the real antagonist is the less conspicuous Christine (Anne Massey).
Christine and Nora (Claire Bloom) were the best of friends years before the play opens. I believe that it is at the peak of their friendship that a jealousy begins because, while Nora prepares to marry the man she loves, Christine is preparing to deny the man she loves and marry a man that can support her family financially. The two women grow apart mainly because Nora, by her own admittance, cannot find time to write to her dear friend even after Christine has been left a widow.
When Christine returns, it seems as if she envies Nora in a way. She notes her beautiful children and cozy seeming life. As the events of the story unfold and Nora's deception comes out, Christine is put in a position to help her friend. Since Christine was once the object of Krogstad's affections, she has the power to persuade him to obtain the letter he wrote to Nora's husband explaining her fraud. She fully understands the power she possesses and even offers to help Nora by talking to Krogstad and asking him to retract the desperately vengeful disclosure. By making this offer, she becomes Nora's only real hope of salvation.
When Christine visits Krogstad, she reveals that she wants to marry him, and even continue to work and help support him and his children. The dark cloud that hangs over Krogstad seems to lift, and he intends to retrieve the letter. For a moment it seems as if Nora might be saved, until Christine blocks the door. She tells Krogstad to let events run their course and that Torvald's reading the letter is for the best. Although she justifies her actions and it seems as if she has Nora's best interests at heart, I believe she acts out of some type of jealousy.
Although Nora seems to welcome her reason for freedom and self-discovery, Christine's supposedly helpful but actually jealous actions have quite torn a family and a marriage apart.