In Act IV, MacDuff has gone to England in order to get English military support and convince Malcolm to return to Scotland. He has suspicions that Macbeth killed Duncan and knows that his is a tyrannical ruler who must be stopped.
Remember that after his father's death, Malcolm fled to England so that whoever killed his father wouldn't be able to kill him. When MacDuff shows up saying that he wants to bring him back to Scotland in order to restore his role as king, Malcolm is not sure if he should believe him. Malcolm lies about himself in order to test MacDuff's loyalties and ensure that they lie with Malcolm. Before he will join MacDuff, he wants to make sure they are on the same side- a side that wants what is best for England.
It is myself I mean, in whom I know
All the particulars of vice so grafted
That, when they shall be open'd, black Macbeth
Will seem as pure as snow, and the poor state(60)
Esteem him as a lamb, being compared
With my confineless harms.
I grant him bloody,
Luxurious, avaricious, false, deceitful,
Sudden, malicious, smacking of every sin
That has a name. But there's no bottom, none,
In my voluptuousness Your wives, your daughters,(70)
Your matrons, and your maids could not fill up
The cistern of my lust, and my desire
All continent impediments would o'erbear
That did oppose my will. Better Macbeth
Than such an one to reign.(75)