Malcolm wants Macduff to use his grief to fuel his desire for revenge against the guy Malcolm wants to get rid of. It's basically the same thing Claudius does with Laertes' grief in Hamlet.
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.
No, it was Malcolm who said it in Act 4 Scene 3No, Malcolm did in Act IV Scene 3, when talking to MacDuff.
stop being so indecisive and take immediate action.
Malcolm tells Macduff that if he had complete power, he would be even more tyrannical and corrupt than Macbeth, making him an unfit ruler. This is part of Malcolm's test to see if Macduff is truly loyal to Scotland and not just seeking power for himself.
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.