This is a rather curious scene in this play where Malcolm shows his own character and ability to manipulate and test his subjects. In response to Macduff's claims that Macbeth must be deposed, Malcolm presents himself as an even worse king than Macbeth, saying, for example, that:
But I have none: the king-becoming graces,
As Justice, Verity, Temp'rance, Stableness,
Bounty, Perseverance, Mercy, Lowliness,
Devotion, Patience, Courage, Fortitude,
I have no relish of them; but abound
In the division of each several crime,
Acting it many ways.
He clearly exaggerates his own "evil deeds," lying to Macduff deliberately to test his loyalty to Scotland rather than to a specific person. When Macduff says that such a king should not be given power, Malcolm says:
Macduff, this noble passion,
Child of integrity, hath from my soul
Wip'd the black scruples, reconcil'd my thoughts
To thy good truth and honour.
Malcolm therefore quite coldly has tested Macduff to work out where he stands and if he can be trusted. The main function of this scene appears to be to assess the moral forces present in the drama that are opposed against the evil of Macbeth, however, rather worryingly perhaps, Malcolm shows himself able to manipulate others and test them for his own motives and reasons, which could either be shown as a sign of political maturity or a sign of potential evil to come.
In this scene, Malcolm deliberately lies to Macduff because he wants to test Macduff's loyalty and trustworthiness. Specifically, Malcolm falsely presents himself as a man with extreme sexual desires who is driven by the "cisterns" of his lust. In addition, he tells Macduff that he is very greedy and would deliberately cause arguments ("quarrels unjust") with his subjects so that he could confiscate their lands and "wealth."
No matter what Malcolm says, Macduff's loyalty does not falter. He defends each of Malcolm's lies and becomes even more determined to overthrow Macbeth.
That Malcolm lies to Macduff demonstrates that he is a cautious person who wants to know who he can really trust. He wants to surround himself with the right people because he knows that this is the surest way to guarantee success against Macbeth.