In Act 4 scene 3 MacDuff has traveled to England to convince Malcolm to return to Scotland and fight for his rightful place as king. Malcolm, who fled to England shortly after the murder of his father King Duncan, is hesitant to believe MacDuff. He knows that he used to support Macbeth, and is worried that MacDuff has come to England in order to trick him and possibly kill him.
To test MacDuff, Malcolm lists his many vices and states that he believes he would be a worse king than Macbeth.
I will grant you he is bloody,
Luxurious, avaricious, false, deceitful,
Sudden, malicious, smacking of every sin
That has a name. Only there's no bottom, none,
In my own evils. Your wives, your daughters,
Your matrons, and your maids, could not fill up
The cistern of my lust, and my desire would overcome
All international factions
That did oppose my will. Better Macbeth
Than such an one like me to reign over Scotland
MacDuff's response, that Malcolm is the rightful heir to the throne, persuades Malcolm that MacDuff only wants what is best for Scotland. With this, Malcolm agrees to go fight.
Macduff, this noble passion,
Child of integrity, has wiped the
Black scruples from my soul, and reconciled my thoughts
To your good truth and honor. Devilish Macbeth
Has sought to win me into his power
By many of these thoughts, and modest wisdom keeps me From over-believing haste.