Which phrase in act 1 of Henrik Ibsen's A Doll's House suggests that Krogstad is prone to engage in crime and may, in fact, be a criminal?

There are various phrases in act 1 that allude to Krogstad's criminal past. Helmer, in fact, explicitly mentions it when he tells Nora that Krogstad "forged someone's name." Earlier in the act, Rank says of Krogstad that he "suffers from a diseased moral character." This sums up the reputation that has stuck with Krogstad since the crime. And Krogstad himself says to Nora, "I daresay you know, like everybody else, that once, many years ago, I was guilty of an indiscretion." The word "indiscretion" is a euphemistic reference to the crime and implies that Krogstad doesn't think the crime was a particularly serious one.
Krogstad also implies that the crime he committed was committed out of necessity. He says that he "had to do something." He also complains that he has been held back ever since by the criminal reputation which has followed him. It has held him back in his career and it has also cost him the woman he loved. Much of the pity we might have for him, however, is of course diminished somewhat when he decides to blackmail Nora.