Brutal and treacherous though it was, Macbeth's killing Duncan had a certain cold logic to it. Duncan was king, and Macbeth wanted to be king, so the only way Macbeth could make that happen was to have Duncan assassinated. So he did. The killing of Lady Macduff and her children, however, is a completely different story. Their murder has no justification whatsoever—not even from a political angle. It's nothing more than a cold-blooded act of revenge in response to Macduff's sudden flight to England, where he intends to join up with other Scottish rebels.

The murder of Macduff's family illustrates just how much of a tyrant Macbeth has become. He's chosen to take his stand with the forces of darkness (as personified by the Weird Sisters), and as such he has effectively sold his soul to the devil in return for the Scottish throne. If the violence that Macbeth displayed in getting rid of Duncan showed us his ambition, then the murderous vengeance he unleashes upon Macduff's family provides us with a terrifying glimpse of a dark soul corrupted by absolute power.

Before this, Macbeth has killed to get rid of anyone in his way. This killing of Lady Macduff and her son reflect the deterioration of Macbeth's morals because these offer no challenge to his quest for the throne. It is a useless killing done because of Macbeth's increasing paranoia. Ironically, this contributes to his downfall because of his involvement with the "killers" and Macduff's fury.

In Act 3.4, Macbeth says, "I am in blood / Step't in so far that, should I wade no more, / Returning were as tedious as go o'er." Now that Banquo has been murdered, Macbeth believes he must continue killing to secure his throne. The death of Lady Macduff as well as of her son and the entire court at Fife reveals a side of Macbeth unseen before. Now he is killing innocent women and children, people who pose no threat to him and who cannot defend themselves.

Macbeth is anxious about Macduff's absence from Scotland because the king fears that Macduff may try to overthrow him. By killing Macduff's family, Macbeth hopes to lure Macduff home again so that he can kill the thane of Fife and eliminate him as a threat. Macbeth's choice, however, to wipe out Macduff's family is the vicious act of a desperate man. Conditions in Scotland do not seem to reflect a successful reign for King Macbeth. Macduff has indeed left the country to seek military assistance from the English with the hope that he can also persuade Malcolm to return and assume his position as the rightful king.