In A Doll's House, what in Krogstad's first appearence on stage, and in Dr Rank's remarks about him, indicate that the bank clerk is a menace?

In the first scene of Henrik Ibsen’s play A Doll’s House, we are given a few clues that Krogstad’s character is a menace.  When Krogstad first comes on stage, we see that his appearance makes Nora very uncomfortable.  In a strained whisper she asks why he has come to see her husband.  When Krogstad answers that it is only about his position at the bank, we see that Nora seems relieved.  All of this indicates that Nora and Krogstad share some sort of secret together and that Krogstad holds some sort of influence over her.  Plus, when Dr. Rank first speaks of Krogstad, he calls him “morally diseased” [Act I], which also helps to indicate that Krogstad is somehow a menace