What reasons does Malcolm give for suspecting Macduff in Act 4, Scene 3 of Macbeth?

There are definitely several reasons why Malcolm fears and distrusts Macduff that Malcolm states in his speeches found in lines 10-31 of Act 4, Scene 3. More specifically, Malcolm fears that Macduff has been sent by Macbeth to murder him because Malcolm remains an obstacle to Macbeth's abilities to keep the crown.

One reason is stated in lines 14-16. Malcolm points out that they once believed Macbeth to be an honest man; more importantly, Macduff once "loved [Macbeth] well" (15). Furthermore, Malcolm argues that Macbeth has not yet hurt Macduff personally, since this is prior to their knowledge that Macbeth has ordered Macduff's family to be murdered. Malcolm sees Macduff's love for Macbeth and the fact that he bears no grudge against Macbeth as an excuse for Macduff to feel inclined to do Macbeth's bidding. Macduff may have been sent by Macbeth to kill Malcolm, like a "weak, poor innocent lamb" (18).

When Macduff argues that he is no traitor, Malcolm's next argument is that, even though Macduff appears to be good, he cannot understand why Macduff has come to England to help him; plus, even the "brightest" angels have fallen from grace. Hence, Malcolm's essential argument for this fraction of his speech is to say that appearances can be deceiving and that even the appearance of goodness can't be trusted.

A final reason why Malcolm distrusts Macduff is because he sees no motive for Macduff leaving his family in a vulnerable position to come and help Malcolm, unless of course he was commanded to do so by Macbeth. He can't imagine that Macduff would have less of a motive to remain with his wife and child than he would have to come and help Malcolm.

Hence, all in all, Malcolm simply feels it is too good to be true that Macduff would leave everything behind, leaving his family so vulnerable that Macbeth could send someone to murder them, all for the sake of helping Malcolm regain the crown. Instead, he fears that Macduff must be being loyal to Macbeth and must have been sent by Macbeth to murder Malcolm.

What are Malcolm's initial feelings toward Macduff in Act 4, Scene 3 of Macbeth?

In Shakespeare's Macbeth we see an interesting interaction between the rightful heir to the throne, Malcolm, and the nobleman MacDuff in act 4, scene 3.

Malcolm and MacDuff have each fled to avoid Macbeth’s murderous rampage. As royalty, Malcolm is naturally suspicious of anyone who might stand to gain from betraying him. Early in the act, Malcolm is unsure of MacDuff and probes him to test his loyalty, saying:

He [meaning Macbeth] has not touched you yet. I am young; but something

You may deserve of him through me; and wisdom

To offer up a weak, poor, innocent lamb

To appease an angry god.

Malcolm is saying the MacDuff might betray him to get on Macbeth’s good side. MacDuff immediately tries to defend himself with:

I am not treacherous

Nevertheless, Malcolm feels the need to test him further. So he pretends to be similar to Macbeth; his own version of the “angry god.” In the next excerpt he is saying that if he can defeat Macbeth, he will be an even crueler and more bloodthirsty tyrant:

It is myself I mean, in whom I know

All the particulars of vice so grated

That, when they shall be open’d, black Macbeth

Will seem as pure as snow, and the poor state

Esteem him as a lamb, being compared

With my confineless harms.

In other words, as bad as Macbeth has been, he will seem like a lamb compared to the speaker, Malcolm. Of course, none of this is true. After going on like this for a little longer, Malcolm is satisfied that MacDuff is not loyal to Macbeth, and confesses that he was merely making sure of his loyalty.

So we have seen Malcolm's attitude toward MacDuff go from suspicious to assured over the first half of the scene. 

Later in the scene we see perhaps the most poignant moment in the play when MacDuff learns that Macbeth has murdered his wife and children.

How is Malcolm and Macduff's relationship presented in Macbeth Act 4, Scene 3?

Malcolm tests Macduff’s loyalty in Act 4, Scene 3, and Macduff passes the test and proves he is a loyal supporter.

When Macbeth kills Duncan, Malcolm flees to England.  He knows he will be suspected if he stays, and instead he can use England as a staging area to develop an army of supporters still loyal to him.  Macduff follows, suspecting that Macbeth might have killed Duncan.

Malcolm is suspicious of everyone.  He feels like he can trust Macduff, but he needs to make sure.

Be not offended;

I speak not as in absolute fear of you.

I think our country sinks beneath the yoke;(45)

It weeps, it bleeds, and each new day a gash

Is added to her wounds. (Act 4, Scene 3)

Malcolm gives Macduff all of the reasons why he would not be a good king, including his age and inexperience.  He would be lustful and violent, and generally untrustworthy.  Macduff does not take the bait.  He stays staunch and true, weeping over Scotland’s fate with the bloody tyrant Macbeth.  Malcolm relents.

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)

In the end, Malcolm realizes that he will make an excellent king.  He is intelligent and honest, and his love of country speaks for itself.  While his brother Donalbain flees and does not bother to help, Malcolm puts all of his efforts into returning his family to the throne and freeing the kingdom from Macbeth’s cruelty and ambition.

This scene marks a turning-point in the Macbeth-centered script.  We see that Macbeth is doomed, because there is someone else ready to take him out.  Macduff and Malcolm have both been wronged, but they are not revenge-focused.  Each of them is more interested in returning the beloved homeland to a noble and honest king.

How does the interaction between Macduff and Malcolm in Act 4, Scene 3 of Macbeth indirectly involve Macbeth?

Act IV, scene three occurs in England, where MacDuff and Malcolm wait outside King Edward's palace.  The premise of this particular act is that Malcolm hesitates to place his trust in MacDuff, so he attempts to test MacDuff's love of Scotland by pretending to have numerous terrible vices.  Here is where Macbeth enters the conversation, because as horrid as Malcolm pretends to be, MacDuff is sure that he cannot be as bad as Macbeth:

"Not in the legions
Of horrid hell can come a devil more damn'd
In evils to top Macbeth" (IV.iii.63-65).

Shakespeare uses MacDuff and Malcolm's conversation to reveal the building opposition to Macbeth; Macbeth's character certainly feels involved in Act IV, scene three, because he is one of the central topics of discussion between Malcolm and MacDuff, especially after MacDuff learns of his family's deaths at the hands of Macbeth's henchmen.  Macbeth's evil presence darkens the end of the scene as MacDuff bemoans his family's undeserved fate and vows for revenge:

"Bring thou this fiend of Scotland and myself;
Within my sword's length set him; if he 'scape,
Heaven forgive him too!" (269-271).

In Macbeth, what are Malcolm's character traits?

Duncan's son, Malcolm, is rarely seen in the beginning of Macbeth (as is Malcolm's brother, Donalbain), but eventually proves himself to be smart, honest, and royal.  After Duncan is murdered, Malcolm and Donalbain both agree to leave Scotland quite quickly which, one must admit, looks a bit suspicious.  However, a bit later in Macbeth, Macduff himself goes to England to find Malcolm and ask him for help in restoring just rule to Scotland.  Malcolm is worried about Macduff's motives, wonders if Macduff has conspired with Macbeth, and is a bit disgusted that Macduff would leave his family to flee to England for this interview.  Therefore, Malcolm tests Macduff with an obvious lie.  Malcolm swears that he adores women, land, gems, and hatred:  all things a true king should despise.  Macduff exclaims in hopelessness; therefore, Malcolm knows Macduff's thoughts to be honorable.  Ironically, it is Malcolm's very first lie that proves him to be so honest!  Malcolm also later suggests Macduff confront Macbeth because of Macduff's murdered family.  Of course, at the end of Macbeth, Malcolm proves to be the perfect King of Scotland in that he wishes for absent members of the military to be present, urges those close to him to mourn their families, births a new era where great military leadership will be rewarded, and (most importantly) agrees to rule Scotland only through God's grace. 

In Macbeth, what are Malcolm's character traits?

Macbeth is overly ambitious, prone to persuasion, and generally successful.

Once he decides to seek Duncan's place as king, Macbeth's ambition consumes him. Not only does he murder Duncan, but he also kills Duncan's guards. Later, he murders Banquo and then Macduff's entire family. Macbeth transforms from a respected and loyal man early in the play to a ruthless and singularly-focused killer. Leaving all sense of morality behind, Macbeth clings to power even when it is clear that his ambition will be his ruin.

Initially, Lady Macbeth has to convince her husband to seek the kingship. When Macbeth hesitates, she calls him a "coward" and insists that he "screw [his] courage to the sticking place," promising that their plans will be successful. Macbeth follows his wife's guidance, becoming the murderer she hoped he could be. He is also easily persuaded by the three witches, allowing their predictions to guide his expectations and actions. Macbeth often lacks any sense of personal direction and allows the opinions and beliefs of others to alter his decisions.

Despite his lack of morals, Macbeth's plans are generally successful until the very end of the play. He defeats almost everyone who stands in his way and is ruthless in the pursuit of his goals. However, this success comes at a price and exposes Macbeth as being a weak and unfit ruler. Although his dreams are seemingly realized, Macbeth finds that he is miserable; his enemies seek him with a vengeance, and even his wife loses her mental astuteness.

What is the significance of the conversation between Lady Macduff and her son?

The conversation between Lady Macduff and her son is significant, as it shows the intimate link in the play between the political and the personal. It's all too easy for us to become so immersed in the high politics of Macbeth that we overlook the consequences of all this plotting and scheming upon family and individuals.

We might expect Lady Macduff to understand the reasons behind the sudden absence of her husband. She is someone, after all, who must have some idea of what political life in Scotland involves; she certainly isn't naive. But as well as being a member of the nobility, she's also a woman and a mother. It's in those capacities that she feels betrayed and abandoned. Her son, though still only a young boy, also feels much the same way.

There is an interesting tension in this scene between the perspective of Lady Macduff and her son and that of Ross, who attempts to justify Macduff's desertion on the basis of reasons of state. It's Ross who tries to get Lady Macduff to see the bigger picture, as it were, to get her to understand that there are sound political reasons for Macduff's sudden departure. In other words, he's separating the personal from the political.

But Lady Macduff isn't having any of it. As far as she's concerned, her husband is dead to her; and there's nothing in this scene to challenge her judgement on the matter. Ross's defense of Macduff certainly doesn't. His apology for his cousin's actions is brief and largely unconvincing, making us sympathize all the more with an innocent wife and child now cruelly abandoned and left to meet their fates.

What is the significance of the conversation between Lady Macduff and her son?

This short scene from Act 4, sc. 2, shows how Lady Macduff feels about her husband's absence and shows the general current mood of Scotland.  Lady Macduff is angry that her husband has left them in Scotland to go to England.  She implies that things are so bad in Scotland that she feels vulnerable and unsafe with him gone (she obviously has good intuition).  She tells her son that her husband is dead, which, in a way, he is to her right now.  Her son knows better and realizes that his mother is simply angry with her husband and is speaking metaphorically.  Ross, her cousin, tries to soothe her, but she says if her husband loved her and his family, he would not leave them alone.  Later, in the next scene, when Malcolm tests Macduff's loyalty to Scotland, that loyalty is clearer to the audience who has seen how Macduff left his family in harm's way to help his country.

Why is the relationship between Malcolm and Macduff crucial in Macbeth?

Malcolm and Macduff’s relationship is important because they are loyal to Scotland and encourage one another to do what is needed to remove Macbeth from the throne.  As future king and the leader of the army against Macbeth, Malcolm needs a loyal supporter like Macduff.

King Duncan had two sons, Malcolm and Donalbain.  Although Malcolm was young, he was the older son and therefore named Duncan’s successor and Donalbain does not seem to have been verycommitted.  Macduff encouraged Malcolm to be king, and Malcolm tempted Macduff to see if he was loyal.  At the same time, Malcolm encourages Macduff to turn his grief for his murdered family into vengeful hate.

Bleed, bleed, poor country!

Great tyranny, lay thou thy basis sure,

For goodness dare not check thee. (Act 5, scene 3, enotes etext p. 68)

When Macduff tells Malcolm that he is not “treacherous” and Malcolm replies “But Macbeth is” he is reminding Macduff that they have a responsibility to the kingdom and to avenge his family (p. 67).  Malcolm tempts Macbeth by giving him reasons why he is too inexperienced to be king and he’ll be easily corrupted, but Macduff is not tempted.  He encourages Malcolm, leading him to believe he can fully trust Macbeth.

Why is the relationship between Malcolm and Macduff crucial in Macbeth?

Macduff is important to the plot because he is the one who eventually kills Macbeth. His family is important because Macbeth sent the murderers that killed Macduff's family. This was one of Macduff's motivations to fight Macbeth.

Macbeth believed Macduff suspected Macbeth had killed Duncan. By this time, Macbeth was getting very paranoid. With hired murderers, Macbeth killed Banquo and tried to kill Banquo's son Fleance. Macbeth did not stop there. He sent the assassins to the Macduff house, where they killed his family.

When Macduff finds out, he is overcome with grief. He tells Malcolm, who tells him that he needs to be a man.

MACDUFF

… All my pretty ones?
Did you say all? O hell-kite! All?
What, all my pretty chickens and their dam
At one fell swoop?

MALCOLM

Dispute it like a man.

MACDUFF

I shall do so;
But I must also feel it as a man:
I cannot but remember such things were,
That were most precious to me (Act 4, Scene 3). 

Basically, Malcolm is telling Macduff that instead of crying over his family, he should avenge their deaths. Macduff is supporting Malcolm, the king’s heir, who left when his father died but also raised an army to fight Macbeth. Macduff figures he has his best chance of defeating Macbeth by aligning with them. 

Macduff is important because the witches told Macbeth that no man born of woman could kill him. Macbeth interprets that to mean he is invincible. He learns that this prophecy, like the others, is tricky. Macduff gives Macbeth some surprising news as they fight:

MACBETH

… I bear a charmed life, which must not yield,
To one of woman born.

MACDUFF

Despair thy charm;
And let the angel whom thou still hast served
Tell thee, Macduff was from his mother's womb
Untimely ripp'd. (Act 5, Scene 8) 

Macduff was born by Caesarian section, so he is technically not born of woman. After this, Macbeth sort of loses his confidence. It is fairly easy for Macduff to behead him, therefore ending Macbeth’s reign of terror and proving you should not always listen to prophecies.

Why is the relationship between Malcolm and Macduff crucial in Macbeth?

The relationship between Macbeth and Lady Macbeth is important for a couple of reasons.  First, it seems unlikely that Macbeth would have actually gone through with the murder if it weren't for his wife.  In Act 1, Scene 7, Macbeth contemplates all of the reasons he has not to kill the king: Duncan is his leader, his friend, his kinsman, his guest, and so on.  He has only one reason to go through with it: his "Vaulting ambition" (1.7.27).  Therefore, he tells his wife, "We will proceed no further in this business" (1.7.34).  Lady Macbeth, however, is not so easily dissuaded from her purpose; she proceeds to verbally abuse him, telling him that he'll have to "live a coward in [his] own esteem" for the rest of his life, calling him "green and pale" (1.7.47, 41).  She berates and insults him, saying that he isn't really a man if he doesn't stick to their plan, and, eventually, she convinces him to recommit to it.  Thus, we might say that Macbeth never would have committed the murder if not for the Lady Macbeth. 

Moreover, their relationship acts as a sort of yardstick by which we can measure their change in the play.  By Act 3, Scene 2, Lady Macbeth no longer seems to be in the loop.  Macbeth has planned the murder of Banquo and Fleance without her counsel, even without her knowledge.  In fact, when he tells her how to treat Banquo at their dinner party later, he's lying to her because he knows that Banquo will not be there (because he'll be dead).  Lady Macbeth is no longer happy, and neither is her husband, but they don't seem to commiserate in the way that they used to do.  She is upset that they've worked so hard only to live in "doubtful joy," and he feels that "To be thus is nothing, / But to be safely thus" (3.2.9, 3.1.52-53).  Neither is content, and perhaps if they worked together, they could figure out a solution, but they do not seem to speak honestly to one another anymore.  It's a really significant change from Act 1.

By Act 5, Lady Macbeth is so distraught that she has begun to walk in her sleep, having terrible nightmares and flashbacks to the night of Duncan's murder.  She even seems horrified, now, by the monster she has created out of her husband, lamenting, "The Thane of Fife had a wife.  Where is she now?" (5.1.44-45).  Macbeth ordered the brutal murders of Lady Macduff and her children, for no other reason than to wound Macduff.  There was nothing to be gained by their deaths as there was with the murders of Duncan and Banquo.  Macbeth has simply become so brutal that he acts without her knowledge and in ways, now, that not even she can condone.  

Her eventual suicide is yet another clue as to how far apart they've grown.  We see no conversation between them at all after Act 3, Scene 4 (the banquet scene), and the sheer lack of interaction between the couple further indicates how much they've changed.  Lady Macbeth used to be his "partner of greatness," and she seems almost nothing to him now (1.5.11).  He instructs the doctor to "Cure her" of whatever ails her but never once goes to her himself (5.3.49).  In many ways, they have switched roles: she was once ruthless and hard, and now she has become guilt-ridden and soft; he was once guilt-ridden and has now become ruthless.  Thus, their relationship helps us to see just how much they've changed throughout the course of the play.

Why is the relationship between Malcolm and Macduff crucial in Macbeth?

The relationship between Macbeth and his wife is very unusual for the time in that, certainly in the early part of the play, she is the instigator and drives their fatal plans forward. Lady Macbeth is unusual - if terrifying - in her calculated planning to kill King Duncan in order to make her husband king. She is aware of her feminine humanity and asks to be 'unsexed' in order to be more resilient in executing their plan.

It is important thatMacbeth regards her as an equal rather than a subordinate. In his letter detailing the encounter with the weird sisters he addresses Lady Macbeth as-

my dearest partner of greatness.

When he returns to elabourate on his experience she has a plan in place, and he is directed to follow her lead -

you shall put
This night's great business into my dispatch,

Their unity does not last as Macbeth becomes more paranoid, organising Banquo's murder without telling his wife. Her mental decline shows also that the resilience she called for from the evil spirits was not delivered, or at least cruelly paid for.

Both Macbeth and Lady Macbeth are held in high esteem by Duncan and the court. It is to the Macbeth's castle that the King chooses to go to celebrate the recent military victory. He addresses Lady Macbeth as-

our honor'd hostess!

He is also keen to continue his praise and thanks of Macbeth-

Conduct me to mine host. We love him highly,
And shall continue our graces towards him.
By your leave, hostess.

What is the significance of Malcolm and Macduff's conversation in Macbeth?

The scene between Malcolm and Macduff in act 4, scene 3 of Shakespeare's Macbeth takes place directly after the scene in which Lady Macduff, the Macduffs' children, and their servants are murdered on Macbeth's order.

The irony of the scene between Malcolm and Macduff is that the audience is fully aware of the brutal and shocking murders, but Macduff and Malcolm are not.

Malcolm heightens the dramatic irony of the situation by remarking to Macduff, "He hath not touch'd you yet" (4.3.16). A little deeper into the conversation, Malcolm adds still more to the irony when says he had doubts about Macduff's loyalty. He wonders why Macduff left his wife and children unprotected, unless he had nothing to fear from Macbeth.

MALCOLM: Perchance even there where I did find my doubts.
Why in that rawness left you wife and child,
Those precious motives, those strong knots of love,
Without leave-taking? (4.3.29-32)

Both Malcolm and Macduff are despairing of the situation in Scotland under Macbeth's tyrannical rule, but Malcolm is nevertheless cautiously optimistic, believing that righteousness and his kingly rights will overcome Macbeth's tyranny.

MALCOLM: Angels are bright still, though the brightest fell.
Though all things foul would wear the brows of grace,
Yet grace must still look so. (4.3.25-27)

Malcolm urges Macduff to join him against Macbeth, but he confides to Macduff that he doubts his own nature and that he fears he would be a far worse King than Macbeth has shown himself to be.

MALCOLM. ... But for all this,
When I shall tread upon the tyrant's head,
Or wear it on my sword, yet my poor country
Shall have more vices than it had before,
More suffer and more sundry ways than ever,
By him that shall succeed.

MACDUFF. What should he be?

MALCOLM. It is myself I mean, in whom I know
All the particulars of vice so grafted
That, when they shall be open'd, black Macbeth
Will seem as pure as snow, and the poor state
Esteem him as a lamb, being compared
With my confineless harms. (4.3.50-62)

Macduff assures Malcolm that no matter how bad a king Malcolm might be, there is no greater tyrant than Macbeth. Malcolm argues that he has no good qualities whatsoever and finally convinces Macduff that his many uncontrollable vices would ultimately destroy Scotland.

Macduff tells Malcolm in no uncertain terms that he's not only not fit to govern Scotland but that he's not fit even to live.

MACDUFF. Fare thee well!
These evils thou repeat'st upon thyself
Have banish'd me from Scotland. O my breast,
Thy hope ends here! (4.3.125-128)

Impressed by Macduff's honesty, integrity, and heartfelt rebuke of him, Malcolm confesses that he was simply testing Macduff and that he's not at all the villain that he portrayed himself to be. Macduff, usually not at a loss for words, is dumbstruck at this revelation.

MALCOLM. ...Why are you silent?

MACDUFF. Such welcome and unwelcome things at once
’Tis hard to reconcile. (4.4.152-254)

If the audience has forgotten for a moment the horror of the scene preceding this scene with Malcolm and Macduff, Ross comes to tell Macduff about the murder of his family and his entire household.

Malcolm commiserates with Macduff and encourages him to turn his grief to anger against Macbeth and to be enraged by the murders. Macduff resolves to take the fight to Macbeth.

MACDUFF. ...[F]ront to front
Bring thou this fiend of Scotland and myself;
Within my sword's length set him; if he 'scape,
Heaven forgive him too! (4.3.269-272)

How is Malcolm initially characterized in Macbeth?

We first meet Malcolm in act 2, scene 3. In this scene, he and his brother Donalbain find out that their father has been murdered and Macbeth has been made king. Malcolm's first words, in the confusion, are an aside to his brother, in which he questions why they shouldn't claim the throne as Duncan's sons and heirs:

Why do we hold our tongues,
That most may claim this argument for ours?

However, after some conversation with his brother, Malcolm changes his mind and decides to proceed more cautiously. He realizes that it is dangerous to stay in Scotland and that he doesn't want to be part of Macbeth's faked sorrow over Duncan's death:

Let's not consort with them:
To show an unfelt sorrow is an office
Which the false man does easy. I'll to England.

This characterizes Malcolm as a man who thinks things through and acts flexibly and wisely.

We don't meet up again with Malcolm until he returns again to Scotland with an army in act 4, scene 3. Here, in conversation with Macduff, he pretends at first that he is a much more evil person than Macbeth, saying,

It is myself I mean: in whom I know
All the particulars of vice so grafted
That, when they shall be open'd, black Macbeth
Will seem as pure as snow.

Malcolm goes on to say that he is so greedy that he would try to seize the lands of his nobles and cause quarrels between them so that he could take even more of their goods. He says, too, that he would generally divide people from one another, destroying the unity of the world:

Nay, had I power, I should
Pour the sweet milk of concord into hell,
Uproar the universal peace, confound
All unity on earth.

He says he has no use for the kingly virtues, such as justice, mercy, humility, and patience. He paints a picture of an evil, narcissistic ruler: one planning to use the crown recklessly for personal financial gain while enhancing his own power through creating hate and division. He is not this person at all, but he wants to test Macduff. He wants followers who are first and foremost loyal to Scotland, not him, and who will put their devotion to their country ahead of devotion to a leader. When Macduff does finally recoil in horror from this depiction of a bad leader, as any sane person might, Malcolm reveals what he is really like: a caring person of integrity who would make an excellent king.

Malcolm shows himself to be a person of great honor, yet one who can use deception wisely to promote the greater good of Scotland.

How does Shakespeare portray the character Malcolm in Macbeth?

Macbeth is a drama about usurpation, and the degree to which that act of betrayal shatters the political order, resulting in tyranny. From this perspective, Macbeth himself should be understood as an aberration, and a distortion of political norms. Malcolm, as the legitimate heir to the throne, plays a critical role within the play's thematic structure, as a foil to Macbeth and as an embodiment of legitimate monarchy, as opposed to the tyranny embodied by Macbeth.

While Malcolm is depicted as noble and virtuous, he is also quite shrewd. After Macbeth's murder of Duncan, both Malcolm and his brother Donalbain recognize the precariousness of their own situation, and thus go into hiding. Later, Malcolm builds an army while in England in order to take the throne that is his by right. However, even as he is gathering allies, he is careful to test for potential spies and flatterers. We see this when Macduff approaches him, and Malcolm claims various vices for himself, pretending to be a terrible candidate for a king. In so doing, he can ascertain Macduff's own personal character.

As the play concludes, Malcolm and Macduff defeat Macbeth, restoring the rightful claimant to the throne. Thus Macbeth's tyranny has fallen, with legitimate monarchy, embodied by Malcolm, finally restored.

Why does Malcolm doubt Macduff's change of character?

Malcolm is suspicious of Macduff's plea to help him overthrow Macbeth for the following reasons.

Firstly, Malcolm states that Macbeth was thought to be honest and that Macduff loved him, so he is really not convinced that Macduff has changed:

This tyrant, whose sole name blisters our tongues,
Was once thought honest: you have loved him well.
 He hath not touch'd you yet. I am young;
 but something
 You may deserve of him through me, and wisdom
 To offer up a weak poor innocent lamb
 To appease an angry god.

The fact that Macbeth has not done anything to Macduff puzzles Malcolm, and he starts wondering if Macduff wants Malcolm to join him against Macbeth only to be lured and killed eventually by the two of them. Malcolm refers to himself as "a weak poor innocent lamb," which means he fears he may be the victim in this affair.

Secondly, he cannot understand why Macduff leaves his family unprotected. If he is truly afraid of Macbeth's evil intentions, he should have not left them behind.

Although Macduff attempts to dispel all these suspicions, Malcolm goes on to test him, saying he is full of vices; he is voluptuous and avaricious and does not possess any of the virtues that kings should boast about. When Malcolm realizes that Macduff is utterly disillusioned and that he bemoans the tragic state Scotland is in, Malcolm becomes convinced that Macduff's change of character is genuine:

 Macduff, this noble passion,
 Child of integrity, hath from my soul
 Wiped the black scruples, reconciled my thoughts
 To thy good truth and honour.

Therefore, Malcolm accepts to join Macduff in order to defeat Macbeth and restore peace in Scotland.