This is a great scene and pivotal in understanding both the characters of Macduff and Malcolm.
Let us start by understanding that Malcolm is afraid of returning to England. He is probably about 14 years old (even though the movies age him to a ridiculous level) and does not want to go to war against a tyrant. He reveals this inital fear through his mistrust of Macduff, whom he cannot believe has left his family in order to help Malcolm.
you may deserve of me through me, and wisdom
to offer up a weak, poor innocent lamb
to appease an angry god.
He is afraid that Macduff is here to trick him back so Macbeth can kill him.
However, as he grows to believe Macduff, Malcolm reveals his fears by pretending he is just as bad as Macbeth, that he is lustful, greedy and ignorant. Macduff sees through Malcolm's ruse, however, but he is saddened that he can never return home.
Finally, Malcolm, feeling bad for Macduff, admits that he does not have what he truly needs to be a king:
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 passages reveals the twelve necessary qualities of a king. However, Malcolm does not have the confidence to believe in himself.