At the end of the act, what does Helmer say has “morally destroyed” Krogstad? What does he say is the “worst danger” of such behavior?

Nora asks about the nature of Krogstad’s past indiscretion, and Torvald reveals that it was forgery. He condemns Krogstad in strong terms for failing to admit it. Torvald admits that he would have forgiven the man had Krogstad owned up to his lie. He suggests that such moral hypocrisy would even infect Krogstad’s family. Torvald makes Nora promise never to plead Krogstad’s case again. He also reveals his intention to fire Krogstad from the bank. Torvald exits to his study. Nora will not allow the children to come into the same room with her. Prompted by Torvald’s comments about moral corruption over forgeries, she is terrified that she will “infect” her own children. Torvald thinks the "worst danger" is Krogstad's bad ways will infect their family.