Malcolm reveals to Macduff that Siward plans to attack Macbeth's stronghold at Dunsinane with ten thousand soldiers.
Malcolm's lie to Macduff reveals his sense of caution and mistrust towards others, as he is testing Macduff's loyalty and intentions. This behavior also demonstrates Malcolm's cunning and ability to manipulate situations to his advantage for the greater good.
Malcolm advises Macduff to go out and fight to help place Malcolm on the throne. All Malcolm sees in Macduff's suffering is an opportunity to gain an ally.
Malcolm is testing Macduff's integrity. If Macduff had embraced Malcolm's worldly, unethical responses, Malcolm would not have trusted him enough to form an alliance with him. When Macduff wails at the imminent hopelessness of Scotland, Malcolm knows he is in the presence of a good man, and he reveals his own "innocence" and stalwart loyalty to Scotland and to God.
Malcolm asks Macduff about his family to test his loyalty and see if Macduff is truly trustworthy. He pretends to be a tyrant like Macbeth to see if Macduff would still support him.
Macduff and Malcolm plan to overthrow Macbeth in England.
Both Malcolm and Macduff believe Macbeth is a tyrant to be overthrown. However, Malcolm is a tyrant himself, if not even more repulsive than Macbeth. Though Macduff is not fully aware of this fact. Malcolm manipulates Macduff into slaying Macbeth and then plea allegiance to himself as the King of Scotland.
By not committing to join Macduff in overthrowing Macbeth, Malcolm gains the opportunity to test Macduff's loyalty and dedication to the cause. It also allows Malcolm to assess Macduff's character and leadership abilities before fully trusting him in the rebellion against Macbeth.
Unlike his father who trusted too easily, Malcolm does not trust Macduff. He does not trust him because he thinks that Macduff may be secretly working for Macbeth. To test this theory he rambles lies giving examples of how he is not fit to be king. However, it is Macduff's polite reaction of disagreeing with Malcolm which makes Malcolm believe Macduff.
Malcolm trusts Macduff because he demonstrates his loyalty to Scotland and his opposition to Macbeth by joining forces with him to overthrow the tyrannical ruler. Macduff's courage and dedication to the wellbeing of Scotland earn Malcolm's trust and respect.
Macduff brings Malcom Macbeth's severed head.
Yes, Malcolm did play a practical joke on Macduff, but it was not meant to be malicious. In Act 4, Scene 3 of Shakespeare's "Macbeth," Malcolm pretends to be unfit to rule, describing his own vices and shortcomings to test Macduff's loyalty and resolve. This ruse is a way for Malcolm to determine whether Macduff is truly committed to the cause of overthrowing Macbeth and restoring Scotland. Ultimately, it shows Malcolm's cautious nature and his need to ensure he can trust Macduff.