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.
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.
He was testing Macduff to see if he genuinely had the interest of Scotland at heart. When Malcolm says "He hath not touched you yet" (IV, iii, 14) it means that he suspects Macduff is in league with Macbeth to take him prisoner and kill him. Malcolm then makes up all his vices to make sure that Macduff wants a good king and not just Malcolm. When Macduff responds that with his vices, Malcolm would make a terrible kind, Malcolm responds that he made the vices up and he knows that Macduff has Scotland's best interests at heart.
Malcolm pretends to be a greedy, lustful, and evil man.
Macduff teams up with Malcolm to fight Macbeth. In fact they meet in England.
Macbeth presents Malcolm with the title of Prince of Cumberland, which is a significant position in line for the throne. This action is a strategic move by Macbeth to ensure his hold on power and to secure the support of the nobles.
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 hesitates to accept Macduff's offer of assistance because he is testing Macduff's loyalty and sincerity. Malcolm wants to make sure that Macduff is truly loyal to him and not trying to trick or betray him. Additionally, Malcolm is feeling distrustful and cautious due to the corrupt and treacherous nature of the world around him.
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.
Malcolm reveals to Macduff that Siward plans to attack Macbeth's stronghold at Dunsinane with ten thousand soldiers.
Malcolm reveals his flaws to Macduff in order to test his loyalty and see if he truly has Scotland's best interests at heart. By pretending to be a worse candidate for king than he actually is, Malcolm is able to gauge Macduff's reaction and trustworthiness.