In A Doll's Houase, is Nora a victim of circumstance or a villain who brings about problems?
I should preface this by saying that I don't really find her a victim, so my analysis here might be a bit biased. I will try to overcome.
In terms of why I might argue that she is at fault, I would suggest that Nora does not activate her voice despite knowing that things are "not right"- There are plenty of opportunities when Nora herself realizes that Torvald treats her in a "thinglike" manner. She fails to act on this until the very end. By allowing this to happen and permitting this state of affairs, she bears some culpability in her predicament. Finally, a reason why Nora might be at fault for her situation is that she truly does not know her husband. She seems to be operating under a pretense of how she thinks of him to be, and only in the end does she realize that her husband truly does treat her in a degrading manner. An open dialogue between husband and wife earlier could have revealed this or prompted change. Her failure to initiate such a discussion rests with her.
In terms of how she is a victim of circumstance, I would begin and end with the social structure of marriage at the time. This conception of marriage "stacked the deck" against an equal conception of power in marriage. This rendering of marriage, in terms of what society deems as "successful marriages", result in men holding most of the power and women being relegated to "trophy status." The favoring of men in this arrangement also stipulates that women have to be loyal to men throughout and do whatever is needed to "keep the marriage intact." This responsibility rests with the women. Thus, women have the pressure to bear for a successful marriage devoid of power. In this setting, how is Nora supposed to activate her voice without risking a great level of inertia or social censure? Even without the institution of marriage going against Nora, the social setting within which she is living is not one predisposed to advocating for the rights of women. There is little to indicate in the play that women can work in the open, demand equal opportunity or equal pay or equality in political representation. If there is such a silence of women's voices entrenched in the social setting, it becomes a tad unrealistic to demand Nora to be a feminist voice in her marriage. Given this social setting and this conception of marriage, the fact that she does activate her voice at the end of the play makes her actions all the more powerful.
Nora is a victim of the male-dominated society of the nineteenth century. To save her husband, Torvald, she borrows money so that he might be able to recover from a life-threatening illness in a warmer climate. Since she could not borrow money without the endorsement of a man, Nora forged her father's signature on the note. From then on, she did what she could to get the money to repay the loan: for example, she took on copying jobs, she bought less expensive clothes for herself out of the money her husband gave her and applied the savings to the loan, and she made Christmas gifts. She could not tell Torvald about the loan, for he frequently comments on the effects of a bad mother on her children. Her decision to keep her secret from Torvald seems reasonable enough; when Torvald reads the letter from Krogstad, the man who made the loan to Nora, he worries about potential damage to his own reputation. He fails to act as if he is concerned at all about Nora. She is, therefore, a victim of circumstance and not a villainess.
This question depends on the individual reader. It is a matter of perspective. My personal opinion is that she is a bit of both, although "villain" is too strong.
Nora borrowed the money under noble circumstances. Torvald was gravelly ill, and they did not have the money to take off for a year. Nora did what she had to do (forgery) to save her husband. Torvald would never have allowed her to make such a decision, and the consequences would have been grave.
However, at some point, after he regained his health, it would have been better to tell the truth. If not then, at least once she was being blackmailed. She created more problems by not coming clean about her actions.
Nora was never malicious in her actions. She believed everything she had done was for Torvald and her marriage. From the fraud to trying to cover her tracks, Nora never meant to be harmful. Had Torvald been a kinder husband, he would have seen this and forgiven her.
A small addition to renelane's answer, would be to consider that Nora says that she hasn't told Torvald about the loan because she wants to keep something in reserve for when she loses her looks. This also doesn't make her a villain, at all, merely a more complex character.