In act 4, scene 3, Malcolm determines to test Macduff's trustworthiness by way of deception. Thus, he claims to be a tyrant in the making, perhaps an even more unworthy candidate for kingship than Macbeth himself, and he makes claim to a series of vices in order to see how Macduff responds.
First Malcolm claims to be overly libidinous, possessed of insatiable lust. Macduff's response shows that he is tolerant of this vice, claiming Scotland to "have willing dames enough." Next, Malcolm claims to be overly greedy, with an insatiable avarice for the wealth of his subjects, but this too Macduff is willing to overlook, so long as Malcolm possesses other virtues to compensate. Here, Malcolm's reply is that he does not possess any of these requisite virtues at all, stating:
But I have none. The king-becoming graces,
As justice, verity, temperance, 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. Nay, had I power, I should
Pour the sweet milk of concord into hell,
Uproar the universal peace, confound
All unity on earth.
This, for Macduff, is the final straw, and he thus rejects Malcolm as entirely unworthy of being king. He is here depicted as falling into sorrow for the future of Scotland. His country has already been in the grip of one tyrant, and now he finds that there is no apparent escape from its misery.
Such a response, however, is precisely what Malcolm had been searching for. As he proceeds to explain to Macduff, he needs to be careful about who he would take into his confidence, given the duplicity of Macbeth himself. Now that Macduff has proven his sincerity and character, Malcolm is swift to correct himself on all of those previous self-denunciations he had earlier raised:
For strangers to my nature. I am yet
Unknown to woman, never was forsworn,
Scarcely have coveted what was mine own,
At no time broke my faith, would not betray
The devil to his fellow, and delight
No less in truth than life.