Malcolm wonders whether Macduff is a paid agent of Macbeth: "You may [deserve] of him through me, and wisdom to offer up a weak, poor, innocent lamb t'appease an angry god (lines 17-20)."
He also questions why Macduff suddenly left his family unprotected to come to England:"Why in that rawness left you wife and child, those precious motives, those strong knots of love, without leave-taking?(lines 33-35).
To test Macduff, Malcolm says he is far worse than Macbeth and that it is Macbeth who will be revealed as the innocent lamb. Macduff states in response that it is Macbeth who is the truly evil character, thus securing Malcolm's trust.
Note: all line numbers are taken from the Folger Shakespeare Library edition of the text.
In Act Four scene three of "Macbeth," MacDuff has arrived in England to try to convince Malcolm to join forces with him against Macbeth. Malcolm has been hiding in England ever since his father was killed, for fear that he would be the next victim. Malcolm has clearly now learned the lesson of the play, that things are not always as they seem. So, he does not trust MacDuff right away. He is not sure that MacDuff is there for pure reasons. He may only want to dethrone Macbeth for selfish reasons, not for a pure love of Scotland. This is why Malcolm tests MacDuff by telling him that he, Malcolm, will be a far worse king than Macbeth. Once MacDuff finally admits that he would not want Malcolm to rule if this were the case, Malcolm confesses he's only been testing MacDuff's motives, and Macduff has passed the test. They then join forces.