Macbeth has at least four motives for ordering the murders of Lady Macduff and all of Macduff's children. Only one child, a boy, is actually shown, but subsequent dialogue makes it clear that there were many others. When Macduff is informed of the atrocity, he says:
All my pretty ones?
Did you say "all"? O hell-kite. All?
What, all my pretty chickens and their dam
At one fell swoop? (4.3)
Macbeth is angry at Macduff for deliberately and flagrantly refraining from attending his coronation and thus ;publicly refusing to acknowledge him as king of Scotland. Then Macbeth is further incensed by Macduff's failure to attend the coronation banquet when all the other nobles are in attendance. Then in Act 4, Scene 1 Macbeth is informed that Macduff has fled to England, and he decides:
The castle of Macduff I will surprise,
Seize upon Fife, give to th' edge o' th' sword
His wife, his babes, and all unfortunate souls
That trace him in his line.
He does this both for revenge and to make Macduff serve as an example of what will happen to the families, titles, and properties of any other thanes who might contemplate following Macduff's example. Macbeth's fourth and last motive for attempting to wipe out Macduff's family and his very existence and memory as a Scottish nobleman is that he was recently warned by the witches' First Apparition:
Macbeth! Macbeth! Macbeth! Beware Macduff!
Beware the Thane of Fife! Dismiss me. Enough. (4.1)