Krogstad actually directly parallels Nora. In the exact same way that Nora is a victim of social injustice and circumstance, so is Krogstad.
Krogstad and Nora are guilty of the same "indiscretion." At the exact same time that Nora's father was ill and dying, her husband was also ill, and the doctors advised her that the only way to save his life was to live abroad for a while in warmer climates. Nora knew she needed money for the trip and so took out a loan. However, in this time period, women were not allowed to take out loans without the consent of a man, either a father or a husband. Nora was supposed to have mailed the papers to her father for him to sign but did not dare inform her father that her husband was in danger of dying while he lay dying himself, as she explains to Krogstad in the lines, "[W]hen he was so ill himself I couldn't tell him that my husband's life was in danger--it was impossible" (I). The result is that Nora forged her father's signature and signed the date two days after her father's death.
Similarly, Krogstad confesses to having committed some sort of fraud, which we learn from Helmer was forging a signature, in order to save his wife's life, as we can see in Krogstad's lines addressed to Nora:
I can assure you that my one false step, which lost me all my reputation, was nothing more or nothing worse than what you have done. (I)
Based off of Nora's own interpretation of what Krogstand is saying, we are further able to interpret that the motive for Krogstad's "false step" was to try and save his wife's life. We see Nora's own interpretation her response, "You? Do you ask me to believe that you were brave enough to run a risk to save your wife's life?" (I).
It is due to Krogstad's fraud that he lost his reputation in the city and had to take any menial job he could find. It is also due to his fraudulent activity that Helmer considers Krogstad a liar and wants nothing to do with him at the bank. However, Nora rightly points out society's unjust reaction to the fraud and argues that motives should be taken into account in matters of the law. Because both Krogstad and Nora commit forgery with the intention of saving someone's life, we can say that both are equally victims of circumstance and unjust laws.