This scene reveals that Macbeth is a cold-blooded killer, willing to kill women and children.
When the play begins, Macbeth is a war hero. He has dignity and respect. By Act 4, he has degenerated to a serial killer. He is now just a murderer. Worried about Banquo, who was his friend, he killed him and his son. He was afraid that Banquo and his heirs would inherit the kingdom because of the witches’ prophecies. He cannot kill Macduff, so he kills his wife and son, who is a child, in Act 4, Scene 2. He doesn’t kill them himself; he sends his henchmen, named simply murderers in the play.
Where is your husband?
I hope, in no place so unsanctified
Where such as thou mayst find him.
He's a traitor.
Thou liest, thou shaghair'd
villain! (Act 4, Scene 2)
The killing of women and children is not an honorable act. It is not an act of war. Where there might be some small justification in killing Banquo and Fleance because of the prophecy claiming that Banquo’s line would be kings, there was no reason to kill Lady Macduff and her son. It was an unforgivable act of coldblooded murder. That Macbeth’s killers would do so indicates the depravity of his reign. He really has become a tyrant. He became king by killing Duncan, and he is doing anything to maintain the throne.
Macduff is a real problem for Macbeth. An honorable man, Macduff has aligned himself with Malcolm, Duncan’s heir and the one who is clearly coming for Macbeth. With Macduff as an ally and encourager, Macbeth should worry. By killing Lady Macduff and Macduff’s young son, all Macbeth has done is strengthen Macduff’s resolve. Malcom tells him to “Dispute it like a man,” meaning to fight Macbeth in battle, and Macduff intends to do so.
But, gentle heavens,
Cut short all intermission; front to front
Bring thou this fiend of Scotland and myself;
Within my sword's length set him; if he 'scape,
Heaven forgive him too! (Act 4, Scene 3)
Macduff will surely avenge the death of his family. There is no doubt. So why would Macbeth do such a thing? Macbeth thinks nothing but of securing his throne. He feels invincible, and he thinks that he does not need to worry about Macduff because the prophecy said that he does not need to fear a man born of a woman. Of course, the contradictory prophecies also told him to beware Macduff, which is why he tried to kill him.
Macbeth is getting more and more off-balance, and it is clear to those closest to him. By the time the enemies are at the gates, how many men will still be loyal to him? Macduff is surely coming for him. Ultimately, Macduff will succeed, as Macbeth loses hope when he learns that Macduff was not of woman born because he was born by Caesarean-section. That is the end of Macbeth. Macduff gets his revenge.