Although Lady Macduff has a brief appearance in Macbeth, her lines really represent her significance in the play.
She is a representative of the death of innocence in Scotland. Her death is also closely linked with Lady Macbeth's descent into madness. Lady Macduff's murder, along with that of her children and servants, is heinous, mad and evil.
Lady Macduff is a good mother, devoted wife and she reveals herself to be outraged by her husband's sudden departure. She is shocked by his behavior. She still has faith in Scotland; she knows she has done nothing wrong. But she realizes that right is wrong in Scotland now and wrong is right, just like foul is fair and fair is foul.
Whither should I fly?
I have done no harm. But I remember now
I am in this earthly world, where, to do harm
Is often laudable, to do good sometime
Accounted dangerous folly; why then, alas
Do I put up that womanly defence,
To say I have done no harm? (4.2)
However, when murderers storm into her house, she is very resolute, as are the people of Scotland during Macbeth's tyrannical reign.
Lady Macduff represents the opinion of the people of Scotland through her dialogue, her honest loving life and her tragic symbolic death.
Lady Macduff, unlike Lady Macbeth, is a strong, intelligent, honest, witty, loving wife and mother. After her conversation with Ross, in which he tells her that Macduff has fled the country ("What had he done, to make him fly the land?"), we see her close relationship with her son when she ironically and figuratively tells him--because she is furious that Macduff has left them alone--that his father is dead (4.2.35).
Lady Macduff: Sirrah, your father's dead;
And what will you do now? How will you live?
We realize she really does not believe her husband is a traitor even though she discusses that possibility with her son: "when our actions do not, / Our fears do make us traitors." The son seems old beyond his years in their talk about the lack of honest men: "there are liars and swearers enow to beat the honest men..." (4.2.62-64).
We see the depth of Lady Macduff's devotion to her husband when she is asked by the murderers where her husband is and she responds, "in no place so unsanctified / Where such as thou mayst find him" (4.2.90-91).
She is also home-loving, courageous and realistic. After the Messenger has advised her to flee and has fled himself, she rhetorically asks, "Whither should I fly? / I have done no harm." She knows that in this world, the good suffer while evil flourishes. Her son is slain onstage; offstage, she is slain after her son.