Oh, a lawyer fellow called Krogstad – you wouldn’t know him. He’s crippled all right; morally twisted. But even he started off by announcing, as though it were a matter of enormous importance, that he had to live.

Dr. Rank describes Krogstad to Nora in this way before Krogstad’s first appearance, setting up perceptions that he is twisted and evil. While Krogstad initially does try to blackmail Nora, he has a more complicated backstory and eventually relents. Krogstad’s motives and circumstances are of no interest to Nora and Torvald at this point in the play; how one appears to others is all that matters.

It never came into court; but from that day, every opening was barred to me. So I turned my hand to the kind of business you know about. I had to do something; and I don’t think I was one of the worst. But now I want to give up all that. My sons are growing up: for their sake, I must try to regain what respectability I can.

Krogstad is explaining to Nora the crime he committed and why he has such a bad reputation. We know that he committed the same crime as Nora, but legitimate firms wouldn’t hire him and he had no choice but to continue in illegal businesses. Krogstad’s experience demonstrates how important one’s reputation is in their world, and how hard it is to succeed without a good reputation.

Mrs. Helmer, you evidently don’t appreciate exactly what you have done. But I can assure you that it is no bigger nor worse a crime than the one I once committed and thereby ruined my whole social position.

Krogstad explains to Nora that forging her father’s signature to obtain a loan was illegal. He then makes clear the difference between Nora and himself: While his social position was destroyed by his forgery, hers is still intact. Krogstad seems to feel no guilt over the fact that he could easily ruin her social position by revealing her crime, and uses the leverage to pressure Nora to persuade Torvald to keep Krogstad at the bank.

But I tell you this. If I get thrown into the gutter for a second time, I shall take you with me.

After revealing the information he can use against her, Krogstad flat out threatens Nora. At this point in the play, Krogstad is not only seeking to restore his reputation, but wants revenge if he cannot keep his job at the bank. While this threat makes Krogstad seem callous, we learn throughout the play how nuanced Krogstad is as a character, supporting the idea that people are neither all good nor bad but fall somewhere along a spectrum based partly on life experience, personality, and circumstance.

Mrs. Linde: You said just now that with me you might have become a different person.
Krogstad: I know I could have.

At the beginning of Act Three, we learn that Mrs. Linde and Krogstad had a romantic relationship before she was married. In this scene, Krogstad indicates that he would have been a different person had they stayed together. As if to demonstrate this truth, Krogstad then returns the forged document. Their words and actions portray a relationship actually based on love in which both people grow and learn.