This is an excellent question. Torvald is a good man, a man of conventions, and obsessively concerned with appearances. He is scrupulously moral, and similarly self-righteous. When threatened with blackmail, he goes into a rage that his wife has disgraced him. Rather than stand up to Krogstad, he knuckles under. He has no real loyalty to Nora; she is what we might now call his "trophy wife."
By contrast, Krogstad resorts to treachery for financial gain. However, unlike Torvald, he has the inner strength to admit his mistake, to forego his vengeance, put aside his anger, and make a fresh start. Clearly, it is Krogstad who is the stronger - and, ultimately, more admirable - character.