For purposes of discussion, it will be assumed that the “massacre” referenced in the question is the murder of Macduff’s wife and children on the orders of Macbeth, Macduff himself having fled to England to both escape Macbeth’s wrath and to consult Malcolm. The murder of Macduff’s family occurs in Act IV, Scene II. The scene opens with Lady Macduff lamenting her husband’s sudden departure, seemingly leaving wife and children to the mercy of a tyrant seriously lacking in that particular quality. Addressing Ross, Lady Macduff ponders the following:
Ross, attempting to rationalize Macduff’s apparent abandonment of his family, attempts to console Lady Macduff by noting the uncertain and perilous times in which they are living, and the fact that her husband’s truest nature must allow for more favorable speculation regarding his reasons for fleeing to England:
My dearest coz,
I pray you, school yourself: but for your husband,
He is noble, wise, judicious, and best knows
The fits o' the season. I dare not speak
But cruel are the times, when we are traitors
And do not know ourselves, when we hold rumour
From what we fear, yet know not what we fear, . . .
Scene II ends with the massacre of Macduff’s family. Scene III opens with Macduff and Malcolm contemplating the dire state of affairs and the threat to all posed by the increasingly tyrannical and unstable Macbeth. Even before learning of his wife and son’s murder, Macduff is under no illusions regarding his enemy’s nature:
Ross, who has arrived in England and meets now with Macduff and Malcolm brings news of growing rebelliousness throughout the country directed against Macbeth’s brutal rule:
When I came hither to transport the tidings,
Which I have heavily borne, there ran a rumour
Of many worthy fellows that were out;
Which was to my belief witness'd the rather,
For that I saw the tyrant's power a-foot:
Now is the time of help; your eye in Scotland
Would create soldiers, make our women fight,
To doff their dire distresses.
After more discussion about the situation, Macduff finally pries from Ross the truth about his family. Ross sorrowfully but truthfully reveals to Macduff the fate of the latter’s family:
The effect of this revelation is to strengthen the resolve of Macduff to confront Macbeth once and for all. His family slaughtered, he now seeks vengeance and is possessed of the capacity to raise the armies, together with the heir to the throne, Malcolm, necessary to wage war on Macbeth: “Let's make us medicines of our great revenge, To cure this deadly grief.”
Lady Macbeth, whose name has become synonymous in modern culture with encouraging treachery and deceit on the part of one’s spouse, is becoming increasingly erratic, her mental state clearly in decline. She utters a phrase that indicates the scale of savagery to which she has been party has begun to take a toll: “Here's the smell of the blood still: all the perfumes of Arabia will not sweeten this little hand. Oh, oh, oh!”
Act V is dominated by Lady Macbeth’s descent into madness, the growing and increasingly militant opposition to Macbeth’s rule, and the climactic lament of Macbeth himself as he displays signs of repentance. Macbeth’s army is disintegrating, and those around him, fearing the coming war, are slipping away. That Macbeth remains convinced of his invincibility, having misinterpreted the witches’ prophesy, is testament to the extent to which he has been guided by delusions of grandeur – delusions that begin to crumble as the battle turns increasingly against him. At the beginning of Scene VIII, he finally acknowledges that all is lost, but will not go quietly, stating, “Why should I play the Roman fool, and die On mine own sword?” He is intent on going down fighting, and Macduff is just as determined to ensure that precise outcome:
The prophecy that Macbeth cannot be killed by anyone born of a woman’s womb is now revealed to be the tyrant’s great miscalculation, as Macduff informs Macbeth that his was not a birth of conventional means, but rather what we today call a cesarean section – extracted from his mother’s womb with a knife. Macduff kills Macbeth and cuts off his head to be displayed to all.
The brutality of Macbeth’s rule had degraded the humanity of all those around him. He himself was so determined in his quest for ultimate power that he didn’t appreciate the nuances of the witches’ prophecies, and his methodology was such that he ultimately emboldened those around him in opposition to his rule. The effect of the massacre of Macduff’s family was to instill in this otherwise peaceable man a thirst for revenge and in others a fierce determination to defeat Macbeth. It also represented the final moral defeat of Lady Macbeth and represented the beginning of the end for Macbeth.