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.