Overall, one can say that Krogstad is a morally problematic character. Whether that means that he's “morally diseased” is another matter entirely. The prevailing morality of the time would certainly have regarded him as such—as expressed by Dr. Rank—but nowadays we would probably just say that he's deeply flawed.
In any case, whichever expression we decide to use to describe Krogstad, there's no doubt that he's very far from being a fine, upstanding member of society. For one thing, he commits forgery before the events of the play take place, a crime that his community is well aware of. Additionally, he assists Nora in her own fraudulent efforts to obtain a loan from the bank using a forged signature. Not wishing to be fired from his job by Nora's husband, Torvald, Krogstad attempts to blackmail Nora into interceding on his behalf.
Morally reprehensible though this behavior may be, it's not entirely clear that it comes up to the standard of what constitutes a sickness. What Krogstad is doing is wrong, to be sure, and profoundly so, but as with Nora he appears to be behaving the way he does out of fairly honorable intentions. He wants to keep his job at the bank in order to protect his family's reputation rather than for his own benefit.
The parallels with Nora are not difficult to spot. She forged her father's signature on a loan application document so that she could raise some money to pay for her husband's rest and recuperation. As with Krogstad, she wasn't thinking of herself but of her family.