In Act 4, Scene 3, Malcolm does not trust MacDuff; he is completely skeptical of MacDuff's loyalties or feelings. Malcolm feels as though he must be extra careful, because it was extremely possible that MacDuff might still be loyal to Macbeth and could have been sent by him to kill him. In order to get a clear picture of MacDuff's motives in coming to England, Malcolm devised a clever test--he pretends to be even worse than Macbeth and lists numerous made-up vices and then waits to see MacDuff's reaction. MacDuff is not put off by Malcolm's pretense; no matter what how bad he may be, Malcolm is still a better choice for king than Macbeth, the usurper. Malcolm is the rightful heir to the throne, and Macbeth must be defeated.
In Shakespeare's Macbeth we see an interesting interaction between the rightful heir to the throne, Malcolm, and the nobleman MacDuff in act 4, scene 3.
Malcolm and MacDuff have each fled to avoid Macbeth’s murderous rampage. As royalty, Malcolm is naturally suspicious of anyone who might stand to gain from betraying him. Early in the act, Malcolm is unsure of MacDuff and probes him to test his loyalty, saying:
He [meaning Macbeth] has not touched you yet. I am young; but something
You may deserve of him through me; and wisdom
To offer up a weak, poor, innocent lamb
To appease an angry god.
Malcolm is saying the MacDuff might betray him to get on Macbeth’s good side. MacDuff immediately tries to defend himself with:
I am not treacherous
Nevertheless, Malcolm feels the need to test him further. So he pretends to be similar to Macbeth; his own version of the “angry god.” In the next excerpt he is saying that if he can defeat Macbeth, he will be an even crueler and more bloodthirsty tyrant:
It is myself I mean, in whom I know
All the particulars of vice so grated
That, when they shall be open’d, black Macbeth
Will seem as pure as snow, and the poor state
Esteem him as a lamb, being compared
With my confineless harms.
In other words, as bad as Macbeth has been, he will seem like a lamb compared to the speaker, Malcolm. Of course, none of this is true. After going on like this for a little longer, Malcolm is satisfied that MacDuff is not loyal to Macbeth, and confesses that he was merely making sure of his loyalty.
So we have seen Malcolm's attitude toward MacDuff go from suspicious to assured over the first half of the scene.
Later in the scene we see perhaps the most poignant moment in the play when MacDuff learns that Macbeth has murdered his wife and children.