Why does Torvald refuse to let Mr. Krogstad keep his job?

In Act I, Torvald enters after Krogstad leaves his home. 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.