In Act 4, Scene 3, Malcolm tempts Macduff, trying to make sure he is loyal. Macduff then finds out that his family has been killed, and is saddened. Malcolm strengthens his resolve.
In this scene, Malcolm and Macduff are at the king’s castle in England, where Malcolm has fled when Macbeth killed his father. Malcolm begins by suggesting they find a place to sit and cry, but Macduff suggests fighting instead. Malcolm then asks Macduff about Malcolm, and Macduff reples that he is not treacherous. Malcolm replies that Macbeth is.
Malcolm is trying to see if Macduff is loyal to him. He tempts Macduff, suggesting that he is not fit to govern for a variety of reasons, including his age and inexperience. He suggests that he might become greedy.
[Were] I King,
I should cut off the nobles for their lands,(90)
Desire his jewels and this other's house,
… I should forge
Quarrels unjust against the good and loyal,
Destroying them for wealth.(95) (Act 4, Scene 3, p. 69)
Macduff does not take the bait. He is determined to be loyal to Malcolm, and convincing. Malcolm finally agrees that he can trust Macduff.
Macduff, this noble passion,
Child of integrity, hath from my soul(130)
Wiped the black scruples, reconciled my thoughts
To thy good truth and honor. (Act 4, Scene 3, p. 70)
Although it may seem like Malcolm has changed, and just decided to be a good king, he really was just playing. He wanted to see what Macduff would say if he laid out all of these dramatic faults.
Macduff only falters when Ross arrives to tell him that his family is dead. Malcolm steps up, telling him to “dispute it like a man.”
Be this the whetstone of your sword. Let grief(265)
Convert to anger; blunt not the heart, enrage it. (p. 74)
Macduff does not actually change, but finding out that his family was slaughtered by Macbeth does upset him. He is tormented by guilt and grief.
Although it seems like there is a lot of changing in this scene, the characters are actually just re-affirming their attitudes and renewing their resolve to get ready to go and fight Macbeth.