By Act IV of Macbeth, Macbeth has killed Duncan and Banquo. He has been visited, according to his own imagination, by Banquo's ghost and he visits the witches for confirmation that he is secure in his position. The first apparition warns Macbeth of Macduff which confirms Macbeth's fears and, having heard that Macduff has gone to England, Macbeth intends to kill "all unfortunate souls that trace him in that line" (IV.i.152-153). Macbeth knows he must act quickly while he is still resolved to kill Macduff's whole family.
In scene ii, Lady Macduff questions why her husband left in such a hurry. She wonders why she should patiently wait for him when he did not wait around to ensure his family's safety before leaving. Ross cautions her that she can't understand what motivated Macduff to leave but she feels abandoned. She is not concerned with the greater good when her immediate family is under threat.
There is irony in her words to her son whom she tells that "he is dead" (38). She suggests that he may as well be dead because he has failed his son. She is not concerned that Macduff may die but that his own son will suffer and his line may even die out itself. Macduff's actions, intended to save the throne, may result in him losing his own family, having the opposite effect than Macduff could possibly have intended. Ironically, in being a loyal subject to his king and country, Macduff has failed in his duties as a father.
Lady Macduff knows her husband is alive and that he is not a traitor to Scotland. She is angry with him (he's a traitor to her and the children) because he left her and his children to go to England to team up with Malcolm. She feels his place was there protecting her and the rest of his family. It is ironic because she was right - she is killed, as are her children. Had Macduff been there to protect them, they may have survived the attack.
Lady Macduff tells her son that his father is a traitor and is dead. The irony tis that he is very much alive, working to rid the country of the real traitor, who is Macbeth. Following her conversation with her son, she is murdered by Macbeth's men, and in the next scene, 4.3., Macduff mourns the death of his family, this giving him more courage and determination to fight the tyrant, Macbeth.