In this scene before the King's palace in "Macbeth," Malcolm, suspicious of Macduff, tells him that
This tyrant, whose sole name blisters our tongues,/Was once thought honest: you have loved him well; [and] may deserve of him through me; and wisdom/To offer up a weak, poor, innocent lamb/T'appease an angry god. (IV,iii,12-14)
Here Malcolm suggests that Macduff may betray him to Macbeth, but Macduff replies that he is "not treacherous." Malcolm apologies: Let not my jealousies be your dishonors" (IV,iii,29). He then goes on to say that he speaks not just in fear of Macduff, but also in fear of England, for he would not be a good king:
...yet my poor country/Shall have more vices than it had before,/More suffer, and more sundry ways than ever,/By him that shall succeed. (IV,iii,46-48)
Then, he deprecates himself, saying that compared to himself "black Macbeth/Will seem as pure as snow (IV,iii,52-53), but this is said only to test Macduff. When Macduff refutes his statements, telling Malcolm to "fear not yet/To take upon you what is yours (IV,iii,69-70) and that his vice can be "portable,/With other graces weighted" (IV, iii,89-90), Malcolm, sensing Macduff's despair when he cries,
Fare thee well!/These evils thou repeat'st upon thyself/Hath banished me from Scotland. O my breast,/Thy hope ends here! (IV,iii,11-113)
explains that he does not mean what he says in disparagement of his own character. He tells Macduff that after he has thus shown such passion, he knows Macduff to be a "Child of integrity," so he "adjures/The taints and blames I laid upon myself," because he has been testing Macduff's loyalty and sincerity. Shakespeare has employed this discourse to demonstrate that Malcolm is a good, humble man who should be king.
Using another technique, Shakespeare has a doctor speak to Malcolm about people with scrofula, a skin disease called "the king's evil" because it was believed that it could be cured by the king's touch. This passage anticipates the news brought by Ross to Macduff that his wife and children have been murdered. Malcolm tells Macduff that they must give the "king's cure," so to speak, to Scotland, and rid it of the disease of the evil Macbeth:
Our power is ready;/Our lack is nothing but our leave. Macbeth/Is ripe for shaking, and the pw'rs above/Put on their instruments. Reveive what cheer you may./The night is long that never finds the day. IV,iii,236-240)
You may wish to read the etext sited below because a modern English version is given beside it that you may more easily understand.