The end of Macbeth Act IV, scene iii, may be the first and only time that two characters are honest with each other in the entire play. The outcome is that the rightful heir of Scotland, Malcolm, and Scotland's most loyal thane, Macduff, decide to restore Scotland to her former self.
In the first half of the scene, Malcolm tests Macduff's loyalty by pretending to be a worse tyrant than Macbeth. Some critics (Susan Snyder) think Malcolm may be telling the truth here--that he will continue the reign of blood that Macbeth has begun. Macduff laments this but does not take action: he must realize that Malcolm is play-acting.
Later, after Ross informs Macduff that his family has been slaughtered, Macduff grieves and then transforms into an agent of revenge. Malcolm seems to take sadistic satisfaction in the news--he relishes slaughter--which validates the above fear that Malcolm is as cruel as Macbeth.
Overall, the scene is troubling no matter how it is read. Scotland seems doomed for disaster regardless. We fear that Malcolm may become Macbeth or that the cycle of revenge will continue to repeat itself. Certainly, Malcolm's pact with the English army will present problems for Scotland in the future.
Macduff meets Malcolm in act 4 sc.3. Malcolm had earlier escaped from Scotland to take shelter in the court of the English king. Macduff crosses over to England to make Malcolm aware of the reign of terror unleashed by Macbeth. He wants Malcolm, the legitimate heir to the throne of Scotland, to return and oust the evil rule of the tyrant. Macduff is certain that all people of Scotland, including the women and youth, would mobilise themselves under the leadership of Malcolm.
Malcolm, however, doubts Macduff's bonafides; he may be an agent of Macbeth trying to lure him back to death. After long deliberations, Malcolm agrees to lead a military campaign against Macbeth.The news of the brutal killing of Macduff's family hardens him and also strengthens his bond with Malcolm.